It's not because of the size difference, it's because Ali fundamentally wasn't very polished. He couldn't handle Norton's jab, how would he handle Klitschko's nuclear bomb jab detonating on his face repeatedly? Ali dropped his hands too much, and despite what 3 second clips on Youtube and his soundbites make people believe, he wasn't that hard to hit.
He always threw his right hand out when he jabbed, so he'd get counter jabbed all night long, and most importantly, he loved to lean away from punches, so with Klitschko being a master at feinting the jab, let's just say it would't be a tough fight for him.
This isn't to say Ali isn't one of the greatest, it's more to say Klitschko is also one, and the fight favors Klitschko stylistically. Much more modern, polished, and disciplined (boring) fighter.