When he transformed from a skinny kid to what he is today, steroids was what I thought too. He denies it of course, but I still believe that's the secret. Yes, he works out hard, but I think he's had some help.
He kinda reminds me of the old Charles Atlas ads they used to have in the comics ... basically bulking up over night. Some of the older ones here will probably remember those ads.
If he'd slow his swing down from 140 to about 120 he'd last a lot longer. And probably hit the ball just as far.