Education request (top/bottom vs versatile)

Because people are very different. Sometimes a smaller guy prefers to top, sometimes a big guy prefers to bottom. Pornography and fiction both create a fantasy where the bigger men will always be dominant, virile and tops, while smaller guys will be sensitive, delicate and submissive bottoms. That does not correspond to reality, however. Once one becomes sexually active, they realize sexual preferences can be very different from how one chooses to be affectionate, with all kinds of combinations happening.

Also, most gay men in real life are versatile and enjoy doing both in sex, and that doesn't indicate anything about how they act in relationships.

In fiction, what happens is heavily tied to the author's personal preferences and fetishes, unless they're a really good storyteller and able to create something that doesn't correspond to what they're looking for necessarily.