This drawing is sorta based on the oneshot The Fattest Beatle. There's a part in the story where his cheeks are described as being bigger than Paul's bum. So here he is bragging about that fact.

John: My cheeks are rounder than your bum!

Paul: You what now?!

Well I know that nothing in the world is bigger than Paul's ass so if John claims his cheeks are rounder than they must be enormous!