>>264033They... pad.
Seriously, though, they waste space all in the name of allowing you to download some files and not others. In other words, they assume that in, say, a torrent of 16 JPGs, you want to download only a couple of them, and are okay with downloading a "padding file" of 300-500 k per JPG in order to not have to download any JPG's that you don't want.
It's one of those cure-worse-than-problem things.