I like qBittorrent as well. But you can't have everything all the time.
rTorrent is the thinnest without any webUI running. Only a few megabytes (at most!).
So yeah. Maybe a Transmission-daemon (due to RPC) would be lightweight as well.
But qBittorrent falls out of that since we need Qt, we need a web server and everything.
Which are not heavy on desktop and normal servers, but VPS is really limited on everything (especially RAM ...but CPU is not powerful either usually... )