The latter's a PDF with quite a lot of images, which will bulk it out a fair bit (
http://www.docs.sony.com/release/PRS505.pdf), while an SF novel downloaded from Baen, for instance, seems to typically be ~0.5 to 0.8Mb in .lrf format (less, if you strip out the cover image), just to provide some notes from the 'other end' of the range!