Why are iMovie files so large?

I've edited a 40mb, 13 minute mp4 file down to 10:30, but when I export it in Media Browser (or 'Export Movie'), the mp4 file size is 105mb (Medium size). I want to use it in Keynote, and this size is acceptable, but why is a shorter video file 2.5 times larger after editing?

Many thanks,

Lorimer Burn