It isn't? The answer is simple: your friend's going to be right, and you're wrong. You've already discounted the possibility of atypical body types and sizes in the group. Even after being pulled out of your universe and into another more than once, you've limited yourself by the rules that applied to you before instead of right now, which is, frankly, stupid of you.

[ also he's like. 80% sure he could do it but we're not talking about that and he's not admitting it. ]