currently each icon gets at most two representations: one used at 1x scale and one used at all other scales. If you want an icon to be used at 16x16 in one place and 20x20 in another UI surface, if you want to tweak the icon for display at 20x20 then you have to add a separate icon definition for it. It would be nicer if a single icon definition incorporated 16x16, 20x20, 32x32 and 40x40 for this situation. One extra benefit is that we'd reuse the 20x20 specific definition for a 16dp icon @1.25dsf, instead of using the 32x32 and scaling down.
see for example the help icons added here[1] or ash::kSystemMenuBusinessIcon etc.
[1] https://chromiumcodereview.appspot.com/2327943002/
Comment 1 by tdander...@chromium.org
, Sep 15 2016