It's a skill to learn how to work against pressure and how to protect the ball, and size doesn't make you not have to know how to do those two things.
He has to learn.
Big men just don't learn low post offense like they once did.
He only benched 185 nine times in the combo so he is weak by NBA standards. I think him getting pushed around was about his lack of leg strength. PP got low and use a low center of gravity. One has to realize that Melo is a neophyte at basketball.
That being said he looks stronger than he did when he first got drafted. It may take a year or two to get NBA strong.
Well he's 6'7 ... I guess by nba standards that's normal but in reality yes he's very big and strong and uses his body well
I've have seen 6'7 guys who are weak and not strong like say Reggie Miller. Height does give leverage but not always strength. I am over 6'7" so PP is not a big man to me. I am probably 1-2" taller than that depending on shoes but I am 6'7" in bare feet. It took some lifting and hard work to get stronger but it's not automatic ability of size.
Melo will get there, just takes some time, he is young. A twenty year old is not a full grown man at peak strength. PP could guard LeBron straight up and that is no small feat. That alone ought to show his strength. I have seen him back down Carmelo too, who is bigger. Give him a few years and look out he will be a wall down low.