the thing that makes an IT file small is the allmost the same way why a midi file is small, like a midi file only includes a time table for when it needs to play a instrument. by MIDI the instruments come with the soundcard drivers (i think) and IT includes the instruments itself.

if you have 5 hours of the same tick tack tock it will have the same size like it was 5 minutes.

the more instruments you get from a WAV the smaller the filesize. (doesnt sound right but it is think about it)