Its not a matter of head size. And in point of fact stronger neck muscles, halting the motion of your head would make it more likely you'd be knocked out. Think about Newton's laws. Your brain is not screwed in place inside your head. There's some giving room. Which means that as your head moves with yor brain, it bounces around within your head less and thus causes less trauma to the brain. But if you lock your neck and don't allow any motion, all of the kinetic energy that would have bled off as your head spun would be translated right into your brain.
Having an iron chin is more the ability to come back from a reeling punch or kick rapidly. If you get tagged and stumble around for even a moment, its over. Especially if you drop. And that's really not something you can train for. Its more of a genetic chance.
The area of impact also has a significant importance. Getting cracked on the jaw hurts and spins the head heavily, generating alot of kinetic energy. A shot to the temple immediately makes the pupils dialate, causing disorientation. Anywhere in between can cause serious trauma to the skull itself but doesn't jar the brain as much as the two afore mentioned regions.