Ali is an interesting case. Early on, you wouldn't think extremely highly of his chin. Now some of how he handled the incoming leather later on had to do with rolling with shots, using his intelligence to downplay how much the shot bothered him, but it's interesting how it appeared that his chin improved as he got older. And he was visibly hurt by one body shot from Mildenberger....Foreman and especially Frazier used his ****ing body as a punching bag and it appeared to not bother him in the slightest. He finished those fights strong too.
I heard this from a doctor on a tv program about combat sports. He said that when you are dehydrated you lose some "Cerebrospinal fluid" and it takes days for the density of the fluid to return to normal. So in other words extreme weight cuts definitely might make you chinny.
^ This raises a disturbing question: I wonder if it would be possible to inject some kind of viscous solution directly into a fighter's intercranial chamber, to temporarily improve his ability to take a punch? Egads... but I bet Panama Lewis has already tried it.
Some of the old timers would chew jerky or even some inert strip of leather to strengthen their jaw muscles. Repetitive even resistance oriented chewing also strengthens the neck muscles that run along the jaw and connect to the parietal portion of the skull. I think they are the sternoclidomastoid(sp) muscles.