Size is not the problem here. SNEmulDS can play 4 MB games fine (Chrono Trigger, entire Donkey Kong Country series). It's the SA-1 coprocessor chip that comes with SMRPG, which is 3 times faster than the system's CPU itself. This chip is not emulated at all, and will probably never be emulated at a playable framerate.
Plus, like MasterPenguin pointed out, SNES roms cannot be trimmed, at least not effectively.