I dont know how you can say MMA fighters have the same chins as boxers.
Just look at Oliver Mcall, Erik Morales, Shane Mosley (prime) Antonio Magarito and the list goes on and on. Boxers train to be punched in the face through thousands of rounds of sparring, granted they have better defence but the constant punishment they take in the gym is a strong contributing factor to why they can take a punch so well.
An example:
The infamous incident with Luis Resto boxing Billy Collins Jr without any padding inside the gloves, this is very comparable to a 4oz glove.
Billy Collins took his best punches for 10 rounds without falling or getting knocked out, and people are supposed to tell me that MMA fighters chins (who get knocked out by an uppercut without any leverage via Silva) are even remotely comparable?
aha sure keep dreaming
oh heres a picture of Billy Collins Jr after the fight. Boxers are real fighters, not homos who love to wrestle on the floor.