One of my male friends can carry me around pretty easily (it freaks me out), but I'm barely 5'3" and lightly-built and he's 6-something. My boyfriend, who's well under 6' and probably similarly strong, cannot comfortably carry me in a princess carry. So there's something about leverage, I think.

And firemen carries are uncomfortable for the person being carried, and consequently unromantic. But I don't get the fetish about carrying people.