DSCompress (I think I was the one to upload it to the
downloads section) can do zip and gz formats.
The whole wait thing mentioned coupled with the relatively poor compression (when you compress a rom most of the space is just that which you would save trimming save a couple of roms which the devs left stuff like unicode where ASCII would be fine).
As GBA roms tend to be written elsewhere on a cart these days they are a good candidate for compression (as I recall Chuckstudios' 3 in1 app has it built in).