Perkins is 6-10 (don't be fooled by the public address announcer announcing him in the starting lineup as 6-11; they do that with Varejao now too, as if they both grew an inch in their mid-twenties). He's more of a wide body than a big body, relatively speaking. Because of how wide he is, physical he plays and intimidating he looks, there's a tendency to call him a 7-0er, but he's not.
People have a tendency with basketball players 6-10 and up to refer to them as 7-0ers, but you'd never hear someone call a 5-10 person 6-0. Because so few players are legitimately above 6-10, my theory is every inch above 6-10 is significant. Bynum, despite his outdated listing, is actually 7-1, not 7-0. So really, there's a three inch height differential between Bynum and Perkins. So the reason Perkins looks relatively small next to Bynum is because he is.
If Bynum played on any other team in the league, he'd be All-Star caliber and throwing up 20/10/2. There might not be a true center with better touch around the rim than him. Oden, like Howard, is more of a defensive center. Bynum, like O'Neal, is a presence defensively because of his combination of length/mass, but he's more of an offensive center.
He get's lost in the shuffle, but I don't disagree with what Breen said in game 2, that Bynum could one day become the most dominant big man in the game.