After all my years of watching boxing I think facial width determines the ability to take punches without getting dizzy.
Iron jaw doesn’t exist. It’s just the difference between a guy who fights always braced and ready to take a punch (and has enough energy/strength left to absorb the force) and a guy who doesn’t. Which is why it’s the punch you don’t see that hurts you i.e when you have your defenses down. Thus conditioning = chin.