UFC fighters usually don't have the level of chins boxers do. Thus they have a whole 'mixed' arts to compensate.
GSP will take it to the ground.
Others use their wrestling, BJJ and Judo to avoid a shot.
Some use pure kicks to hold back punches.
It's also not just chin but durability and reflexes to roll shots.
Big show got a bloodied nose after 2 mayweather punches. Just sayin'
Look at the HW UFC champ, pure hungry boxer type style.