Are you talking about files you have downloaded via gnutella network or, files you have chosen to download via bittorrents?

With torrents, the incomplete file reserves the space on your HDD.
With Gnutella downloads, it depends what parts are downloaded first. Sometimes the beginning, a chunk in middle & a small part of end are downloaded so it shows the file as being much larger than it really is .. basically doing the same as above ... reserving the download space needed to finish it. This also tends to be an issue with particular types of video. I can't recall which one it is whether it's mpg or avi.

Best idea is to re-search the file using same terms you originally used. Try to find more sources. Then at end of search, right-click search tab & choose find more results.