I think the top/bottom thing is mostly mental attitude, but I think it's easier for a top guy to bottom. Once a top gets past the psychological change of becoming submissive, then all he has to do lie back and "receive". On the other hand, in order for a bottom guy to be a top, 2 things have to happen: he must psych himself up to be dominant AND ALSO sexually be able to rise to the challenge.
From just a physical standpoint, ANYONE can be a bottom, but to be a top (even a bad one) always requires the added degree of performance.
I'm a big guy that loves to top another masculine guy-- any size. People usually see me as a top, primarily because of my size (6'5", 220 lbs). And I think that I got used to living up to that expectation: being a good top and delivering it.
But here's my secret: I ACTUALLY LIKE BOTTOMING, TOO. It's hot. But for some reason I would never, never, ever admit that to another guy-- EVER!!
I will never allow another dude to think that he can "just have" my ass. He's got to work for it and then "take" it from me. And if he's man enough to get across that finish line, then I will throw my long hairy legs and big size-14 feet in the air-- let him turn my ass into pussy and be his bitch. I'm 40yo and there's only 5 men that got to see me in that position-- and they damn sure never expected it either.
I love to be the power-driver, but there's a part of me that secretly loves to be power-driven just as much.