Using that logic, shouldn't Fedor have lost to Hong Man?
Shouldn't Cro Cop have been owned by Bob Sapp?
Shouldn't BJ Penn have been crushed...many times?
Also, don't forget, A. Silva is 6"2.
Even then, skill > size any day. Just because he can't toss Lesnar around like a rag doll doesn't mean he will let Brock get a hold of him. And being heavier doesn't make you less likely to be knocked out.
Please do not compare the skill set of Bob Sapp or HMC to guys like Kongo, Gabe, AA, Couture, ETC. It's an insult. When people have skills and a remarkable size differential, they'll win 95% of the time.