What about the shape and width of the head?
If your head is wider and more round shaped It would give the brain more room to move without hittin the skull Wall. If you have a very thin head the brain would hit the wall much easier. Alot of these fighters have much squarer jaw lines as well. Can you prove this wrong by thinking of any fighters with solid chins and thin heads? Or Round heads and suspect chins?
Bj penn, Anderson Silva, Chris Leben, David Tua, Mark Hunt, rampage all Have round wide heads.
Jonathan Goulet, Kendall Grove, Amir Khan etc... Have thin heads.