shr-max-image-proportion

shr-max-image-proportion is a variable defined in `shr.el'.
Its value is

0.9

Documentation:
How big pictures displayed are in relation to the window they're in.
A value of 0.7 means that they are allowed to take up 70% of the
width and height of the window. If they are larger than this,
and Emacs supports it, then the images will be rescaled down to
fit these criteria.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.