In my opinion, the number one factor seems to be the size of the head. Leben, Rampage, Cabbage, Hunt, Fujita and Ortiz all have big heads and those guys have all taken vicious shots to the dome, many without even falling down.
ive never been knocked out by a punch or anything as of yet. but when ive takin the most damage is when i dont see it coming. or i see it at the last second and only flinch but not out of the way. so i think yes when you see the strike first and prepare for the damage the extra neck muscle will help. but when you arent ready for the strike it obviously does more damage. i think this was shown in fedor and hendo. he didnt see it coming.
Strong neck, big chin/jaw, being a ginger are all things that are supposedly helpful with not getting KO'd.
Just to follow up on my previous post:
KO's are mostly caused by one of two things.
First is "hitting the button". The flash KO. Thats hitting the chin in the perfect spot which drives the jaw bone below your ear into a cluster of nerves bunched up there. The pain caused from the bone jamming into the nerves creates an overloading in the brain which then causes the body to temporarily shut down. This is observed as an instant of paralysis followed by a longer time of disorientation and a lack of coordination.
The second type of KO is the shots that lead to a "shaking" of the brain that causes unconsciousness due to not enough blood getting to the brain. Shots to the jaw, behind the ear, etc. are all places that cause the brain to shake more.
Where a strong neck comes in to play is the weaker your neck is, the more your head will snap when the KO shot is landed. More snap equals a higher amount of brain shaking (yes I know shaking sounds stupid, I cant thing of another word to describe it). Hunt, Cabbage, Fujita are all examples of the fat necks that keep the brain from shaking more when the head is struck.
Where a bigger chin comes in to play is on the flash KO's. The more bone you have between the chin and the nerve cluster at the ear, the more energy that is diffused before it reaches the cluster. Once a certain amount of force is used, a perfect shot to the chin will still pretty much drop anyone, especially if the person being hit is moving into the striker as that increases the force being used (think of a car hitting a parked car, then think of two cars racing towards each other and hitting, one crash will much more devastating than the other). Crocops pointy little chin is an example of how this theory works in reverse.
Finally we have the ginger theory. Its been tested over the years and shown that red headed people have a higher tolerance to pain. Dentists and Anesthesiologists first noticed that it took more to knock out or dull the pain in red headed people than everyone else (some credit this to having no soul). Aspirin, pain killers, etc. all work less on gingers. Gingers have a higher pain threshold. As a result, it is said that it takes a little more to overload the nerves in a flash KO punch as a result. This could be why someone like Leben is able to seemingly take punches that others cant.
As I said a few times, this all helps with the KOs. It isnt foolproof. All of the examples above have been hit with the perfect shot and all have gone down as a result. But those traits above help with the shots that arent the most accurate and would probably knock out someone with a normal jaw, normal neck and normal hair. ;)