Dear Marcel,
At tex.stackexchange.com a moderator thinks it's a bug in xdvipdfmx.
If so, where to report it?
The bug may not be in xdvipdfmx, but it may be in XeTeX itself.
See below.
%
% xelatex test.tex
% sizes are calculated internally by xetex itself,
% and they are incorrect
%
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\includegraphics[width=8cm]{test-cmyk300.jpg}
\includegraphics[width=8cm]{test-cmyk72.jpg}
\includegraphics[width=8cm]{test-rgb300.jpg}
\end{document}
%
% latex --shell-escape test.tex
% dvipdfmx test
% sizes are calculated by dvipdfmx (extractbb) and they are correct
%
\documentclass{article}
\usepackage[dvipdfmx]{graphicx}
\begin{document}
\includegraphics[width=8cm]{test-cmyk300.jpg}
\includegraphics[width=8cm]{test-cmyk72.jpg}
\includegraphics[width=8cm]{test-rgb300.jpg}
\end{document}
Best,
Akira
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