OK, this is a little odd, but I knew a woman once who was in a relationship with a dwarf. Possibly there's a more PC term for this, and I don't mean this in any way except factual. Anyway, sadly, he used to beat her. I never talked with her about the physical power dynamics in the relationship, but obviously she wasn't being physically overpowered. In this case it was abuse and not all in fun, but he clearly had some kind of power advantage in the relationship that transcended the physical. I certainly don't see any reason that you couldn't freely cede power to someone who couldn't physically wrest it from you.