love how people are suprised at the size of a midi file, think of it like an html file, where each note is a different letter and each letter is assigned a colour (as the voice), and then realise that MIDI is essentially a language/protocol for notating just those two things, along with a few additions, like changing the pitch or adding effects.
combine that with every MIDI compatible device having at least the same 128 General Midi voices (or near enough the same), and you essentially only have to store the sheet music for the computer to play.