In the toolbar most icons are 1.5em x 1.5em which is 24px x 24px in the default implementation. In cases where icons need more padding we have set them to 2em x 2em instead of adding 0.25em padding.
If icons were always 1.5em x 1.5em we could set background-size to contain and then users could change their base font size to scale the whole interface, including icon sizes. Currently setting background-size to contain makes some icons larger than others.