Thanks, I had not noticed this.
On Windows 7 the icons will update to the desired size if you restart LibreCAD.
The Categories toolbar uses QToolButton
which is different from the icons on the other toolbars.
It looks like it changes size up to a certain point.
22 → 32 there is a clear change in size.
If I change it to 42 then I start to see space on the side.
I'll do some more research.