Hearns had that Bruce Lee like power. Summoning all his muscle with each punch. He just had gifted power if you will.
I thougt he fought Ray too early, but nonetheless he messed up Sugar Ray's eye forever. Had he had waited a year or so, and gained some distance (late round boxing) experience he would have been unbeatable. I followed his whole career and saw greatness very early.
With that said Marvin Hagler kicked his arse. Hearns went up in division and paid a price partly because big boys hit harder and can take little boy's punches, at least until the fighter adjusts to the new weight class.
Goes to show you the level and number of great boxers back in the days.