I've been using Deluge... it does the job, but I'm pretty sure it is a load of crap. Program frequently hangs or becomes unresponsive. Never fails to download though... I'm completely indifferent to this experience though as I'm not exactly relying on it for anything.
In the case of qbittorrent, if Windows shipped with Qt by default, you wouldn't be complaining, because the size would be around 2 Mb. . Efficiency is relative. However, the reason Linux (and software in the distros repos) occupies much less space than Windows is that is much more modular and resources are more shared.
