Too many things play a part in genetic potential. Am I saying a natural bodybuilder can get 250-300+ lbs stage weight? No, but there is a huge difference in the appearance of the weight we come in at.
And the rate of muscle gain has a lot of factors that mess with that as well.
Look at Josh Hyaduck, one of the top "young" natural pros. Kids been on a BULK-CUT workout/diet plan since he was 14! FOURTEEN!!!!!!!! Kid prolly hadent even hit puberty yet and he knew what a PCF breakdown was and was eating 4-6 meals a day smashing weights, because the only thing he does... is bodybuild. I know this through personal sources and know people that have hung out with the kid and workedout and talked to him. Not only in those first 4 years (14-18) did he gain stupid amounts of mass, it didn't stop there, kids still growing, at a rapid rate as well. Now he's 23 and entering the prime of his manhood, you don't think he's going to gain more? I bet he does.