Good morning,
I have just noticed that \box_resize_to_ht:Nn forces the given box
into a square instead of scaling the width appropriately. The other,
\box_resize_to_wd:Nn works as expected as it preserves the original
aspect ratio:
\documentclass{article}
\usepackage{expl3}
\usepackage{graphicx}
\usepackage{mwe}
\begin{document}
\newsavebox\picbox
\savebox\picbox{\includegraphics{example-image-a}}
\typeout{orig. size: \the\wd\picbox\space X \the\ht\picbox}
\ExplSyntaxOn
\box_resize_to_ht:Nn\picbox{60pt}
\ExplSyntaxOff
\typeout{\string\box_resize_to_ht:Nn \the\wd\picbox\space X \the\ht\picbox}
\savebox\picbox{\includegraphics{example-image-a}}
\ExplSyntaxOn
\box_resize_to_wd:Nn\picbox{60pt}
\ExplSyntaxOff
\typeout{\string\box_resize_to_wd:Nn \the\wd\picbox\space X \the\ht\picbox}
\end{document}
Kind regards,
Alexander