Actually, the themes are pretty central to the game; they're part of the cutscenes, they're individually tied to specific adventure cutscenes, and by default they are chosen at random when you play quick or custom games. You'd only ever hear one music track if we just packaged a default theme, and that would drive most people absolutely batty. Having extra downloads is also something we studiously avoid, as the vast majority of players simply can't be bothered in our experience!
The reason that the file size is so much larger than a lot of 2D puzzle games is that the music is at a high bitrate (lower bitrates get pretty grating), and the art is at HD resolution (so basically a power of two more pixels -- and storage space -- compared to, for example, Popcap games). All that extra space goes to good use, don't worry! Believe me, if we could save download size it would be in our best interest to do so, but having an HD quality game requires more size, same as having a Blu Ray disc requires so much more space than a DVD.
Hope that helps!
