Since I can use 1 answer as well, I have to respond @nomad. What are you talking about? All you had to do is one quick wiki search and you would know your theory ( which is weak at best) is unfounded. At age 14 Manny ran away and moved to Manila where after breifly living on the streets, he started boxing and joined the Philippine National Amatuer boxing team where his room and board were paid for. He turned Pro at the age of 16 and has been since. He is an Army reservist which other than training for boxing would be his other laboring as you state.
But if that was a case then the U.S. would seemingly have lots of Pros from the Military seeing all the armed forces have Amateur teams. There are a lot of people who do hard labor in the U.S. as well. I think I would lean more to the fact (if not roids) that it's just natural for him from starting at such a young age, and thus spending more years in the gym.