Oh, they're not fixed at 18x18. Your computer is just telling inkscape to use that.

Actually, the sizes are queried from the current theme. The current theme says what is standard. Again, one of the main problems here is that different themes and different distros set different sizes as "standard", so one pixmap does not match all.

I actually was finally able to see more of these problems once I started dual-booting into Ubuntu here. I finally saw some of the things that had been annoying Bulia for quite some time, but never quite got described enough to pinpoint.