Joey Maxim had one heck of a sturdy chin. If Tunney counts as a light heavyweight, he's probably top five.
Bob Foster said the light heavyweight knock out punch felt like the heavyweight jab. IMO, boxing changes at the light heavyweight division. The power, durability and intimidation factors seem to visibly change from middle to light heavyweight.
Yeah, commentators have said that once you get over 200 pounds, the knockouts come that much easier. Lot of mass getting thrown around... You don't suddenly need to be fast and powerful to knock people out. Huge schmoes like Matt Skelton and Danny Williams can easily do it.