I use qBittorrent with sparse files on NTFS and Windows 8.1. I have noticed that files consume vast amounts of space while downloading and then correct when the torrent is finished and removed from qBt.
For example if a torrent is 20GB it will use 35GB during download, measured by the amount of free space reported on the drive. After removing it from qBt I get 15GB of free space back.
My guess is that it's because of sparse files and inefficient use of space on the disk. Anyone else have this issue?