I'm glad this is being talked about I actually pay a lot of attention to this and I think there are a ton of factors. I got here 5 hours after OP posted. and I think pretty much all the comments so far are accurate. Overall I fell like the single most important contributing factor to a KO is a dramatic movement of the skull causing the brain to smash into the interior wall of the brain cavity before the CSF can absorb the shock. What we call "chin" is the ability of a fighter to not let that happen weather genetic or technique and possibly diet. So here I go.

1Head size; To a degree. A larger skull in volume is a larger target. and easier to be hit but not not an actual indicator of ability to absorb punishment. Fujita had a big head, and thick bones. Also had a great chin. I think the thickness of his skull allowed him to absorb damage. Also had a short thick neck Which I also feel plays a role as 1 commentor mentioned. Bigfoot Silva has a big skull but generally loses by KO/TKO if he loses. Tito I can't recall ever being KO'd only TKO'd He has a large skull strong neck. So as far as actual size, not so important. What is important is Skull density, neck length, and neck strength.

2 Jaw&chin; The Jaw and ultimately the chin is the lever, the base of the skull and the top of the spinal cord is the fulcrum point. This is the best place to hit someone to make their brain smash against their skull because the force will be increased as it travels up the jawbone. Imo a round or flat chin is better (Anderson, Lawler, Hunt) than pointed but whats more important is the...um, sideburn? length And chin protrusion which create a longer lever. Jaw strength can also improved to reduce the likely hood of KO.

3Brow ridge; The thicker the better. Although people generally don't get rocked by shots to the middle of the forehead it definitely happens.

genetically a short neck round skull and jaw with prominent brow would be best. If one strengthens their neck and jaw, keeps the mouth shut and their chin down and rolls with the strikes he/she will be very hard to put away. Ive been thinking lately hydration may play roll. Fighters cut a lot of water weight for fights and CSF like everything else in our bodies is mostly water. So I wonder if improper hydration can make a KO more likely.

Im no professional these are just my dumb opinions, you can DV me now