GBA commercial games come in 32, 64, and 128 MB (MegaBit) formats (and possible for a 256 MB but no game has used that yet) So on a 256 MB GBA cart, you can fit two 128's or four 64's or eight 32's. There are programs that trim the roms down if they have extra unneeded padding. Doing that lets you fit sometimes another 32 mb rom on a 256 MB cart. Note the above is MegaBIT not MegaBYTE. GBA carts mesurements are based on megabit which is 1/4 of a megabyte, so a 1 GB GBA Cart is really only 256 MegaByte. And a 256 cart is 32 MegaByte
I have a 256 MB cart and I usually fit 3 to 4 games on it. (more if I wanted a bunch of small games).