Bryan is much smaller than Punk. Punk is the same height as Cena. I don't think Punks match with Brock was as amazing as people make out, and I can't see any way in which Bryan can have a physical match with Brock. I doubt his running knee could even reach Brock's nipple.
Brock's not that tall in wrestling terms - he's 6ft3 - same as Reigns... who Bryan knee'd just fine. He's just built like a freak, which makes him seem taller/bigger. The reason I brought up Punk is because he's a "smaller guy" in the physical sense (not seeming to be "powerful" enough to hurt Brock) and they did just fine... Bryan and Brock are both good enough to mean that size difference wouldn't be an issue at all.