It doesn't support compressed games (do any of them?)
But rom trimming isn't compression, it's snipping of the empty bytes that round the memory off to bit-boundaries: 1,2,4,8,16,32,64,128MB, it's all about addressing memory - if you have a 100MB program, you need a 128MB rom, with 28MB of empty space.
If you're thinking off asking why it isn't filled up with extra content, don't.
Anyway, rom trimming trims off the empty bits (when it's done properly) and rom trimmers have their own thread somewhere. Use the search.