When BitTorrent starts, it allocates space for the entire file(s). That is what you see at startup as the progress bar moves across the screen and the disk drive goes crazy. The reason it does this is because it downloads the file in pieces, and those pieces arrive in an arbitrary order. Unlike http or ftp, which download the file from start to finish, BT downloads it in random order.

Note that newer versions of the client can allocate the space as the transfer progresses, and skip the initial "preallocation" step.