Nothing to do with the actual size of the skull. It has to do with neck and shoulder muscles that can hold your head in place when struck and prevent it from snapping back and forth throwing your brain against your skull.
Nothing can "improve" your chin, but you can help it along, but the biggest thing that goes into creating a great chin is head movement, it doesn't matter who you are, if you get hit clean you're going out.
Now recovery time from getting hit is a whole different ball game. For example, Henderson doesn't have great head movement but when he is hit he recovers very quickly, meaning if if he is wobbled, guys don't jump on him because he isn't out of it long enough.
Think. It Ain't Illegal Yet
"Discipline Hurts Less Than Regret"