Possible solutions:
1) Use a lower bitrate for your mp3s. They'll sound more like crap if you do, though.
2) Convert your mp3s to oggs. They'll receive around a 10% compression. The problem is that they are not streamed in RMXP, so they must be fully loaded until they play. RMVX allows oggs to steam, so this is a good choice for it.
3) Sequence your mp3s into MIDIs. Sure, people will complain, but if the end result makes you happy about the project size, go for it.