Two points I can quickly make here.

1. Some files, particularly some video formats reserve download space. (b) You do not necessarily download all parts of a file sequentially, so you might have the beginning, parts in middle & end, and the info data for that file that tells the operating system how large that file is.

2. There is a known fake bug file floating around that is multiple gigs in size. In the example I'm thinking of, the file mysteriously disappeared. http://www.gnutellaforums.com/334401-post1.html oops .. not good example. I saw another example recently. The one i was thinking of turned out to be a file requesting you to download a codec which is a virus. The torrent community know about this issue.

I hope you checked the file with Bitzi before starting to download it to get user feedback on the file.