Problem with vertical space in Memoir title

Add \strut on each line. This is a box of zero width and height that extends just above and below the baseline which ensures that each line has the same vertical dimension.

Alternatively you could add a \vphantom so that each line has the same vertical dimension. This has the advantage of producing a tighter box, but the disadvantage that you need to specify paramterts which include the characters with the largest depth and height.

Here is a comparison of boxes with no adjustment, using \strut and \vphantom{}:

\documentclass{article}
\begin{document}
\begin{tabular}{c c c}
 & strut & vphantom\\
    \fbox{g}\fbox{f} &
    \fbox{g\strut}\fbox{f\strut} &  
    \fbox{g\vphantom{f}}\fbox{f\vphantom{g}}
\end{tabular}
\end{document}

There's no huge environment; there's the switch \huge:

\documentclass[10pt,a4paper,oneside]{memoir}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[brazil]{babel}

\title{Estudo e modelagem de \\ soluções de otimização discreta para \\ grades horárias}

\newcommand{\subtitle}{
    Trabalho de Conclusão do Curso de \\
    Bacharelado em Ciência da Computação
    }

\pretitle {\begin{OnehalfSpacing} \begin{flushright}\huge\scshape}
\posttitle {\\[1cm]\normalfont\sffamily\Large \subtitle\end{flushright}\end{OnehalfSpacing}}

\begin{document}
\begin{titlingpage}
   \maketitle
\end{titlingpage}
\end{document}