Zdnet believes that the full source code is about 40GB and this is only a partial leak.
http://zdnet.com.com/2100-1104_2-5158496.html
I agree that the file seems to be too small if it fits onto a cd, besides that, don't comments in the source code take up a lot of space (i.e. "I fubared this feature, but maybe someone will get around to fixing it").
note: when I took programming courses, i would but in comments like that for my own personal reference