>> etc/images/icons/hicolor/scalable/
>>
>> Similar subdirs could be added to other images as well
>> because this problem exists for other features, e.g. for
>> subtree arrows in customization buffers, tool-bar icons, etc.
>
> Yup.  Emacs currently auto-scales images using this very er ad-hoc method:
>
> (defun image-compute-scaling-factor (scaling)
> [...]

Meanwhile I tried to scale svg, and the result is not great.
Scaling up makes the image blurred, and scaling down makes the
image smeared.