Having each package being it's own torrent, for only about a few MBs, just wouldn't really provide much benefit, unless there were hundreds of people downloading at once.
What I meant to say here is that if you've got a torrent for a file that's only a couple of MBs big, it will be hard to have a lot of downloaders on the torrent at once; everybody will connect and finish the download very quickly. The result of this is that there will only be one or two downloaders at a time, and bittorrent is actually less efficient in the case of "1 seed, 1 leech" than a standard FTP/HTTP download or whatever yum currently uses to get it's files.
