Brock Lesnar is 6' 3" he isn't a genetic freak...it takes work but getting muscle on a 6' 3" frame isn't a magical thing...it takes work...he (was)is an athlete.
Lesnar's level of muscular definition is extreme. There are very few people, athletes or not who could build that level of lean muscle mass. Lesnar's body fat percentage is probably between 8 and 14% - the level of a much smaller athlete, like a sprint runner or a swimmer.
Compare that to somebody like Mark Henry, who is a former Olympic power lifter. His muscle mass is not lean, he has a large percentage of body fat, as well as muscle that he required in order to gain the body mass needed to increase his muscle size and core strength. Henry is one of the majority people who aren't physically capable of increasing their muscle mass past a certain point, without piling on other body mass.
I'd say as much as 80% of non-naturally regulated competition body-builders have enhanced themselves with either steroids or HGH. Numerous retired bodybuilders have confessed to this, including Arnold Schwarzenegger.
That's why I class Lesnar as an anomaly, because he's naturally been able to build up a hell of a lot of lean muscle mass without taking steroids or HGH. I would also put Ryback and John Cena in this category, although sometimes I'm not particularly sure about Cena. I'll expand on this further if people want to hear it.