Has to be completely psychological for exactly the reasons you mentioned.
That, and the thrill of being in power over someone else, kinda a domination thrill of (temporary at least) ownership.
But if I'm going in for domination (this is pure theory, so walk with me, if you will), I'd think I'd get more thrill from dominating someone who's ordinarily tough to subjugate.
However, I'm a mean Texas broad, so perhaps my domme standards are higher.