Brock was the NCAA Champion, and he probably wouldn't be a UFC caliber fighter if he wasn't fighting in the worst division in the sport. The top 5 or so heavyweight mixed martial artists are legitimately great fighters, everyone else in the division has huge gaping holes in their game. I'd be willing to bet Jon Jones, Daniel Cormier, and Anthony Johnson would be just as successful, if not more successful at heavyweight as they are at light heavyweight.
I think Brock would do okay if he was smaller. He's just a freak of nature and I think it's a little unfair as a comparison. Brock had a great background for MMA and he could cover up his rough spots with pure power and athletic ability. Punk really isn't very athletic so he had nothing to fall back on to cover his weaknesses.