I was setting up GroovyMAME 0.193 tonight.
When you remove "default" the font is smaller, but looks kind of crappy on my arcade monitor. I think it used to look OK in older versions of MAME.
However, if you change it from...
uifont default
...to...
uifont uismall.bdf
...then it looks pretty decent, at least on the handful of games I tested.