As a long-time amateur boxer and very long time amateur athlete I'll take exception to your exception for a bigger head being an advantage in combat sports. For the record my head is slightly larger than average at 59 cm, so this is not a case of "noggin-envy." ;-)

First, having a smaller head does not preclude you from developing a huge neck. Having wide shoulders/a wide back mean you can develop a huge trapezius regardless of how large your head is. In addition, your neck can easily be wider than your head, as you'll see with most athletes regardless of head size.

Second, and more importantly, combat sports is all about hitting and not getting hit. Both of those activities are made more difficult by having a large head, particularly defense (not getting hit). Simple physics makes it clear that moving your head will be more difficult and cost more energy the larger it is. Your head, particularly the danger triangle from your nose to the sides of your jaw is your most vulnerable target. Having more to move increases inertia and canot be seen as an advantage.

Third (but related) and kind of burying the lede: The smaller your head the more difficult it is to get hit in the first place. That should be patently obvious.

Fourth and last: You don't get knocked out because your head whips around per se. You get knocked out because your head takes an impact and your brain sitting a bit loosely in your skull moves at a different speed than your head and hits the side of your skull. If the differential is too large, you'll get "woozy" (concussion) and much more and you'll get knocked out.

You are correct that taking one on the tip of your chin and twisting your head maximizes the twist and thus the possibility to get knocked out. Having a bigger head means having a longer jaw and thus more mass twisting which would increase your chance of getting knocked out. Also the smaller your head the less neck muscle you need to keep your head from twisting in the first place.

We'd need to do a study to actually quantify this, but my experience both as a boxer and a scientist says that the advantage of the increase in enertia from having a bigger head would not outweigh (pun intended) the long list of disadvantages I've described above.

In any case, a very interesting theory I'd be interested in seeing proven or disproven empirically. Thanks for the read!