MP3's are sound files and as such are still pretty large. Whereas midi's and abc are miniscule. There should be some program where you can put the music notation in and just save as a midi or abc.. That's what I do on my PC - just copy the notation into NoteWorthy Composer and then save as a midi (except now I also add lyrics and send joe the NWC file - killing two birds with one stone)

(Sorry - Barfly is actual for ABC tunes - but that would work as well!!!!)