Marquis doesn't get out muscled by threes. He is actually really strong, and one of our best post players (when he has the match up) He played pf/c in college.
He is a good post player due to his footwork, finishing touch, and ability to get his shot off with no space. Strength has nothing to do with it, in fact very rarely does he attempt to use strength to gain advantage or create room (backing down, pushing off). He's a finesse player who relies on his slashing to get inside, not a guy that bangs down low.
You even say he's one of our best post players when he has the match-up....all the more reason not to play him against guys that are bigger than him. He has enough length for the 3 (which is why he's dangerous vs. 2s) but most of these guys have up to 50 pounds on him.