I do not use the Ubuntu desktop or Gnome or GDM3. I do use X and TWM.
Some application icons are HUGE. For example, evince, (I know, gnome).
I replace all the evince.png icons with the 16x16 icon. (backed up first). Evince now has the desired size icon.
It looks like evince used the 256x256 icon. But, replacing just that one did not seem to work. (Maybe I forgot .cache?)
Is the icon to be used coded in the application?