now that I'm off my phone, i'll give a little more in depth opinion. You take a group of people (boxers) who are getting hit from the time they are 10 or so and they grow up that way, they are going to adapt to it somewhat, that adaptation will still be based around a loose aptitude for being able to take concussive punches. The same way a fighter might adapt to their own level for being a puncher. So, in that group, you will have guys who can take great punches and those who can't and all the inbetween. By the time they are pros, that ability is probably at least close to the limits of where it will ever get to. So, a guy with a lousy chin by pro champion boxing standards, (Benitez,Terry Norris) will never ever get an ali like chin by getting hit more, never. Now, you have the guys with great chins, like Chavez, Chuvalo or Ali and they usually are pretty consistent with the punches they can take throughout their careers. Some have said a chin deteriorates, but I don't believe that myself.
Now, we have another group of fighters, who don't want to take hits, use every excuse in the book to avoid taking hits, are petrified of taking hits and think they should only take them when the lights are on. So, by and large, most mma fighters, who haven't gotten hit since the age of ten, play around in sparring for the most part and make excuses for going all out, is it any wonder why Mayorga almost coldcocked a prospective opponent with a slap? Or in the early days of the ufc's guys would get hit by a jab and it was bedtime for bonzo? MMA fighters are just a different species, maybe superior all around if we're talking who could get a W in a real fight but they are just kinda wierd as far as how they think and how they live and how they train. Just this past week, we had some local pro wierdo who used to go around in a super hero costume on the streets to fight crime get busted dealing drugs. Just the kind of wierdo that was commonplace around the mma fighters I knew.