I'm not a major fan of the exclusively top/bottom relationships within fandoms. For some characters it works, but for most, it wouldn't. It really depends of their personalities and the society or culture they revolve around, and then how they'd react to it...
And most would agree on being versatile.
It's insane how some people are willing to take some characters very OOC to keep with the top/bottom idea. That totally throws me off.
I know not everyone's familiar with Final Fantasy 4, but Kain Highwind and Cecil Harvey are a pretty common pairing. Cecil's more feminine, look-wise - but his personality more or less matches it. He's gentle, true... but for hell's sake, he's not a crying bottom. He's more likely to fight for dominance. If you're going to make him exclusively bottom, at least, go for the D/s relationship based on how it allows him to relieve the stress he's under (ruling a heavily militarized country, the medieval society they live in, etc), while this gives Kain the feeling of having even more power (mounting the king... out of everyone in the world...).
At least something that won't make them OOC. Please.
I'll admit, I like height differences. Small bottom / tall top. I dig it. But I won't do it if it clashes with characters' personalities. That's for fandoms anyways, because when it comes to original characters...
Then again, it depends of the culture they live in. In my original story, both guys are pretty tall (6'2 and 6'4). They have roughly the same build too. They're both warriors. Homosexuality wasn't unheard of, and it wasn't much of a big deal - as long as you were the top, because it meant you were powerful enough to mount another guy, therefore powerful enough in combat. Now the bottom was thought of to be less manly, and more at risk of being "defeated" on the battlefield.
So my "bottom" is an over-aggressive swordsman because he's constantly trying to prove his prowess in combat... because his "top" loves the power of mounting another guy and won't give it up easily. Over time, it'll become the main source of frustration. (But I'm nice, I'll have them work it out).