why is there often a correlation between big punchers and poor chins?

Stamina, and the fact that when you throw wide/haymakers your chin is dangling in the air following huge telegraphed punches.

 

Benny Leonard was a very solid hitter though. He had some spectacular OHKO victories and beatdowns in his career. He simply fought with reservation.

 

I think there are few guys with rock solid iron chins. Punchers know they have power. You are at your most vulnerable defensively when you commit to a big power punch, but punchers take those chances to land a big shot so in the process they leave themselves open to countershots which is the best time to hit someone, when their committed to a punch and miss.