NDS roms always have set sizes 32, 64, 128, 256MB
when a game is made that is in a size between 2 of those(example 89MB) it will be filled up with "empty" data
when a rom is trimmed a program tries to cut this empty data out.
this means, a game can be reduced in size by 1-45%
it all depends on the rom you trim.
still, beware as some games use exact locations of data, and trimmed roms have the data at a different location.
so the game can't find it's data.