Fat definitely helps with punch resistance as would a thicker set waist for body shots. But I think Joshua's middle is still extremely strong and durable, I think it's the proportions of the rest of him that give a underwhelming appearance of his waist. Being tall he should be able to avoid too much body damage anyway if he develops into a technical fighter.

Fuck they both look strong though, forces to be reckoned with there. I wonder how they're going to cover that up come testing time lol