Each song is 4 individual music tracks, plus 4 midi files (which are tiny), and they also contain the animation sequences, note charts, and (at least on Rock Band 2) the crowd singing as well. Getting that down to ~40MB on average is no small feat. Death Magneitc (360) is 1.3GB alone, though it includes the GH3 version as well. Still though, even if each song was 20MB, that's only 50 songs per game, which really isn't that much considering there's nearly 300 songs available for Rock Band for download.