Tall fighters tend to have weak chins. Wilder, Hearns, and Imam all have weak chins. Why do you think this is? I think it has to do with their skinny legs. They can't take a punch very well because of them.
I guess the lack of solid neck muscles/traps too. Also, they mostly like to lean back while defending or just keep their hands a bit lower to protect their body, leaving their chin out in the air. If somebody catches them it's lights over. Some of them have good chins though like Margarito, but he didn't exactly fight like a tall fighter.
I am not a wilder fan but you cant really say he has a weak chin when he hasn't been knocked out yet lol.
I don't think its to do with chin so much being weak. Its the style/positioning. As Drachenorden said, they have to lower their hands more to protect a bigger target downstairs. Hearns never struck me as having a weak chin even though hes been sparked a few times. This one will probably irritate a few, but I don't think Amir Khans chin is that bad, or as bad as its made out to be. He becomes wreckless and gung ho, leaving himself wide open for counters.
Hearns chin is mostly criticized for his Barkley and Leonard stoppages. Barkley just cut right through him so fast. And Leonard just landed a hard combo and Hearns was done. It just didn't take a whole lot. Even the Hagler fight Hearns was done after round 1. He legs were rubbery in round 2.
What about ali, holmes, foreman, vitali hopkins? I think its more of a case of sturdy shorter fighters having GREAT chins because they get hit more, therefore they would on average require a better chin to get to the top than a taller rangy fighter
I think Drachen is spot on. Check out this pic of Lamar Odom (NBA Player with a pencil neck) vs. Vitali Klitschko (thick neck, but by no means David Tua or Mike Tyson level) http://www.boxinginsider.com/wp-content/uploads/2009/09/PT24934.jpg Guys with Odom's build will have soft punch resistance most likely, unlike Vitali. Deaontay Wilder looks lanky like Odom, specifically his neck section.
I don't think Hearns had a bad chin. SRL caught him with some perfect shots and he's , imo, one of the top 5 finishers in boxing history. Hagler just walked him down and Barkley was huge for the weight. There's really no rhyme or reason why some fighters have a bad chin. I don't think it's related to height or anything like that - it's like punching power; you either have it or you don't.
I think this whole "good chin / bad chin" thing is 80% about the fighter's style of fighting. Some fighters are better balanced and better prepared to take shots (like GGG) and some aren't. Margarito also comes to my mind here, I think he also has the style where your chin will look better than it is (although his chin was obviously still iron). I think if you have a style where you really plant your feet, hide behind high guard and march forward it's really hard to just get KOd by a flush clean shot. If not for anything else because your chin isn't hanging out. I mean, how many times do you see someone like GGG expose his chin out in a way that it could get hit by some hook? Very rarely. If you fight like this you get finished by accumulation though (in case if your opponents just fires combos while you're in the shell and you can't properly answer back and eventually the shots get through and hurt you), but in these cases no one will say "oh he has a glass chin", instead they'll say "he took hundreds of hard shots, he has a good chin to survive so long". Fighters who are light on the feet on the other hand can't afford to hide their chin all the time though, they need to lean back a bit to be able to bounce away and move and it's hard to really protect your chin this way (other than by moving out of distance). They need to be more fluid in their movement and can't move as freely if they have their chin down all the time or both hands up. So when they do get hit it's likely flush on the chin and it could be that they just aren't properly balanced at that moment as they were moving around instead of being planted to the ground. So the impact will be worse and it will look more like the fighter has a "glass chin" because he will go down by a single shot in spectacular fashion. Just my theory.
KO'd several times in the am's, including once by a LHW. He's also been dropped in the pro's by the morbidly obese Dustin 'Worm' Nichols and down and almost out and saved by the bell vs professional loser Harold 'Hard Roc' Sconiers. His matchmakers have also avoided punchers like the plague. A very sure sign when a team knows their fighter has a fragile jaw.