One issue with MAME, because CRT based TVs(rear-projection) use overscan, parts of the screen can end up cropped (the score labels, credits):
I couldn't find any global setting to fix it, so I started investigating the in-game menu that's accessed by hitting <TAB>.
I found some "stretch" options in the Slider Controls section.
Oddly enough, changing the Horizontal setting in Galaga fixed the vertical cropping.
I gather that's because Galaga using a portrait display, so I started up Rally X which uses a landscaped moniotor
And for it the Vertical setting fixed the cropping issue