Store and reproduce effective graphics scaling

Here is my try: 

The three images have different widths (big, medium and small), but the star has the same size on each. I used \settowidth to get the original image width, and \pgfmathsetmacro to divide the dimentions (answere by Matthew Leingang here helped me).

\documentclass{article}
\usepackage{graphicx}
\usepackage{pgf}

\begin{document}
\newlength{\imagewidth}
\settowidth{\imagewidth}{\includegraphics{img_big}}
\pgfmathsetmacro{\myscale}{\the\textwidth/\the\imagewidth}

\includegraphics[width=\textwidth]{img_big}

\includegraphics[scale=\myscale]{img_medium}

\includegraphics[scale=\myscale]{img_small}

\end{document}

You can then add a \newcommand in the preambule:

\newlength{\imagewidth}
\newcommand\getscale[1]{%
    \settowidth{\imagewidth}{\includegraphics{#1}}
    \pgfmathsetmacro{\myscale}{\the\textwidth/\the\imagewidth}
}

And get the scaling factor for an image using:

\getscale{img_big}
\includegraphics[scale=\myscale]{img_medium}

The following example defines \setscalewidth[<options>]{<image} (and \setscaleheight) to set the scale used by LaTeX when including an image using \includegraphics[<options>]{<image>}:

\documentclass{article}

\usepackage{xparse,graphicx}
\ExplSyntaxOn
  \cs_new_eq:NN \calc \fp_eval:n
\ExplSyntaxOff

\makeatletter
\newcommand{\scalefactor}{1}% Default scale factor
\newcommand{\setscalewidth}[2][]{% \setscalewidth[<options>]{<image>}
  \settowidth{\@tempdima}{\includegraphics{#2}}% Original width
  \settowidth{\@tempdimb}{\includegraphics[#1]{#2}}% Modified width
  \edef\scalefactor{\calc{\strip@pt\@tempdimb/\strip@pt\@tempdima}}% modified/original = scale factor
}
\newcommand{\setscaleheight}[2][]{% \setscaleheight[<options>]{<image>}
  \settoheight{\@tempdima}{\includegraphics{#2}}% Original height
  \settoheight{\@tempdimb}{\includegraphics[#1]{#2}}% Modified height
  \edef\scalefactor{\calc{\strip@pt\@tempdimb/\strip@pt\@tempdima}}% modified/original = scale factor
}

\begin{document}

\setscalewidth[width=0.2\textwidth]{example-image}% Define scale width

\includegraphics[width=0.2\textwidth]{example-image} \quad % Print original image using width= specification
\includegraphics[scale=\scalefactor]{example-image} % Print original image using scale= specification

\end{document}

Once set, the scaling factor is stored in \scalefactor.

With expl3 facilities, you can easily get the scaling factor:

\documentclass{article}
\usepackage{graphicx}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\setscale}{mmm}
 {% #1 is the object to scale, 
  % #2 the width to scale to,
  % #3 the name for the scale factor
  \hbox_set:Nn \l_tmpa_box { #1 }
  \tl_clear_new:c {l_douba_scale_#3_tl}
  \tl_set:cx {l_douba_scale_#3_tl}
   { 
    \fp_eval:n
     {
      round
       (
        \dim_to_fp:n { #2 }
        /
        \dim_to_fp:n { \box_wd:N \l_tmpa_box }
        , 5
       )
     }
   }
 }
\DeclareExpandableDocumentCommand{\usescale}{m}
 {
  \tl_use:c {l_douba_scale_#1_tl}
 }
\ExplSyntaxOff

\begin{document}
\raggedright

\setscale{\includegraphics{example-image}}{\textwidth}{myscale}

\texttt{\usescale{myscale}}

\includegraphics[width=\textwidth]{example-image}

\includegraphics[scale=\usescale{myscale}]{example-image-1x1}

\end{document}