Genetics. Tito had incredibly strong legs, just like Hearns. He could probably squat more pounds than an average heavyweight. Power is generated from the legs, this is basic common knowledge.
People seem to get weights all messed up. Just because people fight in two separate weight divisions doesn't mean the guy fighting in the higher weight division is bigger than the guy fighting in the lower weight division. Hydration levels, muscle tone, bone density, and fat content all play a role in overall weight. Was there any doubt that Victor Ortiz was the MUCH bigger man against Mayweather, despite Mayweather fighting at 147 and Ortiz fighting at 140 for most of the past several years? Marquez looked every bit as big as Manny at 147 despite Marquez being a lightweight as recently as his previous fight. People saying that Manny will steamroll Bradley because Bradley is "smaller" have no clue what they're talking about. Bradley is the naturally bigger guy and you'll see that when they're both in the ring and fully hydrated. I would go as far as to say that Bradley is significantly larger than Pacquiao.