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