If that's your "opinion," then more power to you, I can't hold having an opinion against you. However, here are the very brief "facts,"
Brock Lesnar = NCAA Division I HWT Wrestling National Champion.
Carwin = NCAA Division II HWT Wrestling National Champion.
That's the football equivalent of Lesnar having to have beaten teams like USC, Ohio State, Texas, Oklahoma, Oregon, Ole Miss, Virgina, Florida... etc and Carwin having to defeat football equivalent teams like Grand Valley State University, Glenville State, Carson-Newman State, Chowan College, Livingston College.... who? exactly. Brock reigned in a far more elite level of competition then Carwin in the world of collegiate wrestling and Brock's wrestling records were far more dominating (in spite of being in a superior division).
How much of their past carries over? I honestly couldn't tell you, but if Carwin carried over 100% of what he had in college and Brock carried over 75%, Brock would still be more dominate.
Can Carwin win? Hell yes in my opinion. Does Brock have a wrestling advantage? hell mutherphucken yes, and not in my opinion, but reason based on facts.
As Mullet answered, you can't steady a BJJ black belt on weight alone. There are intentional counter-leveraging you don't "see," and strategic weight displacement both offense and counter/defensively.
P.S. What Lesnar does as far as trying to bear down on the opponent is no different then what Hendo "attempted" to do on Anderson Silva and what GSP did to BJ Penn (with painful success), to Thiago, to Hughes... as non technical in comparison to BJJ as it seems, it's a wrestlers tactic.