unsolicited change of font size in mathjax in reveal.js slides

For some reason, from the 4th and up render of mathjax, it will render at font-size 50%
I don't know why, maybe some config you've overlooked. Anyway, you can override this by adding your own css:

.MathJax_CHTML {
    font-size: 117% !important;
}

117% was the value used for the other elements, so I would just use that value.

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="generator" content="pandoc">
  <meta name="author" content="Jan Heiland">
  <title>H_\infty-control for DAEs</title>
  <meta name="apple-mobile-web-app-capable" content="yes">
  <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
  <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, minimal-ui">
  <link rel="stylesheet" href="https://revealjs.com/css/reveal.css">
  <style type="text/css">
    code {
      white-space: pre;
    }
    
    .MathJax_CHTML {
      font-size: 117% !important;
    }
  </style>
  <link rel="stylesheet" href="https://revealjs.com/css/theme/black.css" id="theme">
  <script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_CHTML-full" type="text/javascript"></script>

</head>

<body>
  <div class="reveal">
    <div class="slides">

      <section>
        <h1 class="title">Riccati-based <span class="math inline">\(H_\infty\)</span>-control for DAEs</h1>
        <p class="author">Lars on stackoverflow</p>
        </section>

      <section>
        <ul>
          <li>Descriptor systems have an ODE part and an algebraic part
            <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
        </ul>
      </section>

      <section>
        <ul>
          <li>Descriptor systems have an ODE part and an algebraic part
            <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
        </ul>
      </section>

      <section>
        <ul>
          <li>Descriptor systems have an ODE part and an algebraic part
            <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
        </ul>
      </section>

      <section>
        <ul>
          <li>Descriptor systems have an ODE part and an algebraic part
            <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
        </ul>
      </section>

    </div>
  </div>

  <script src="https://revealjs.com/lib/js/head.min.js"></script>
  <script src="https://revealjs.com/js/reveal.js"></script>

  <script>
    // Full list of configuration options available at:
    // https://github.com/hakimel/reveal.js#configuration
    Reveal.initialize({

      // Optional reveal.js plugins
      dependencies: [{
          src: 'https://revealjs.com/lib/js/classList.js',
          condition: function() {
            return !document.body.classList;
          }
        },
        {
          src: 'https://revealjs.com/plugin/zoom-js/zoom.js',
          async: true
        },
        {
          src: 'https://revealjs.com/plugin/notes/notes.js',
          async: true
        }
      ]
    });
  </script>
</body>

</html>


set viewDistance parameter to the number of slides you have will solve your problem. Default value is 3, which automatically hide all the following (or the previous ones beyond this limit), setting them to display:none; and this doesn't allocate the space the slide should take

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <meta name="generator" content="pandoc">
        <meta name="author" content="Jan Heiland">
        <title>H_\infty-control for DAEs</title>
        <meta name="apple-mobile-web-app-capable" content="yes">
        <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
        <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, minimal-ui">
        <link rel="stylesheet" href="https://revealjs.com/css/reveal.css">
        <style type="text/css">code{white-space: pre;}</style>
        <link rel="stylesheet" href="https://revealjs.com/css/theme/black.css" id="theme">
        <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS_CHTML-full" type="text/javascript"></script>
    </head>
    <body>
        <div class="reveal">
            <div class="slides">

                <section>
        <h1 class="title">Riccati-based <span class="math inline">\(H_\infty\)</span>-control for DAEs</h1>
        <p class="author">Jan Heiland</p>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

            </div>
        </div>

        <script src="https://revealjs.com/lib/js/head.min.js"></script>
        <script src="https://revealjs.com/js/reveal.js"></script>

        <script>

// Full list of configuration options available at:
// https://github.com/hakimel/reveal.js#configuration
Reveal.initialize({
    viewDistance: 5,
    // Optional reveal.js plugins
    dependencies: [
    { src: 'https://revealjs.com/lib/js/classList.js', condition: function() { return !document.body.classList; } },
    { src: 'https://revealjs.com/plugin/zoom-js/zoom.js', async: true },
    { src: 'https://revealjs.com/plugin/notes/notes.js', async: true }
]
});
        </script>
    </body>
</html>

EDIT :

Another simplest solution is to choose SVG output (no more worries with number of slides)

<link rel="stylesheet" href="https://revealjs.com/css/reveal.css">
        <style type="text/css">code{white-space: pre;}</style>
        <link rel="stylesheet" href="https://revealjs.com/css/theme/black.css" id="theme">
        <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS_SVG-full" type="text/javascript"></script>

        <div class="reveal">
            <div class="slides">

                <section>
        <h1 class="title">Riccati-based <span class="math inline">\(H_\infty\)</span>-control for DAEs</h1>
        <p class="author">Jan Heiland</p>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

        <section>
            <ul>
                <li>Descriptor systems have an ODE part and an algebraic part
                    <span class="math display">\[\begin{align*}
                        \begin{bmatrix}
                        C^{\mathsf{d}}  &amp; C^{\mathsf{a}}
                        \end{bmatrix}
                        \begin{bmatrix}
                        sI-A &amp; 0 \\ 0 &amp; sN-I
                        \end{bmatrix}^{-1} 
                        \end{align*}\]</span></li>
            </ul>
        </section>

            </div>
        </div>

        <script src="https://revealjs.com/lib/js/head.min.js"></script>
        <script src="https://revealjs.com/js/reveal.js"></script>

        <script>

// Full list of configuration options available at:
// https://github.com/hakimel/reveal.js#configuration
Reveal.initialize({
    viewDistance: 1,
    // Optional reveal.js plugins
    dependencies: [
    { src: 'https://revealjs.com/lib/js/classList.js', condition: function() { return !document.body.classList; } },
    { src: 'https://revealjs.com/plugin/zoom-js/zoom.js', async: true },
    { src: 'https://revealjs.com/plugin/notes/notes.js', async: true }
]
});
        </script>