Nobody knows for sure. I suspect it has more to do with the Central Nervous System than anything else. A KO is like your body's emergency shutoff switch. Somepeople just shutoff easier than others. It does seem that once a fighter gets KO'd, it's easier for them to get KO'd a second time. I guy can go a pretty long time absorbing blows but once he gets knocked out, many times it happens easier moving forward. It doesn't mean their skull got smaller, thinner or their neck got weaker. It's more like the CNS learned a response and not implements it sooner. These are guesses. I don't think anyone really, really knows.
This may be the best answer i've ever read. Comparing chins to head size seems so similar to phrenology and I truly doubt that's how it works. I think it could definitely be a cns response, but there is no way to truly know for sure.
Well, I think I have a pretty good chin myself, I have been practicing boxing 6 years now, never hurt to head in sparring, I ve never been knocked down, got hit with some considerable size stones when I was a little boy and have been hit flush shot to the head with the forearm, elbow and a head butt that took off two permanent teeth and never loose my balance. I have a small/medium head size, one of my sparring partners have a zeppelin size head and punches with 14-16oz gloves seems to hurt him every time they land flush on his chin, even soft punches got him hurt. I think this is all about Central Nervous System as one lad above explain. Your brain disconnect to avoid getting more damage. It's also about genetics, my dad has got unconscious from been hit with a tree branch above his head while walking, my maternal grandfather who I look like could take a metal beam to the head without to complain, yes he had a big head, but then again you have people like Josh Taylor with a long neck and little head who has a heck of a chin
Skull Thickness helps especially around the jaw and temples some people have thicker skulls and more muscular around those parts. That and a big thick neck.