Eli Zaretskii writes:
> Can you propose a patch that would implement your suggestion?

It looks like my analysis was wrong.  The problem is rather that
create-image applies a default scaling, when :scale is not given.  This
depends on the font size (only if char-width > 10 pixels), which
explains why it happens on my laptop but not on my desktop machine.  I
guess create-image is targeting image resources like toolbar buttons,
where it may be a good idea to scale the images with the fonts for
so-called "hidpi" displays.  But for image-mode the situation is
different and I at least do not want that behaviour.

I attach three patches, the first is a minimal change that fixes the
problem for me.  The second adds to the doc string of create-image,
because I think that its behaviour is unexpected enough to warrant
highlighting.  The third makes the image-mode transform parameters
buffer-local, because that seems more appropriate and even complies with
the doc string form image-transform-rotation better.  This third patch
changes behaviour, though, so maybe a second opinion is in order.

The patches are against master, I haven't tested if they apply against
emacs-26.  Let me know if there is something that needs to be changed.