You know the other day I was watching the MLB Chanel and they were talking about how even back in the 30's some of the bigger names would start to drop off and then bounce back again. EG: Tris Speaker. At the age of 31 his OPS was .828 and his BA was a .296. But at the age of 32 his OPS was 1.045 and his BA was .388. There were plenty of examples of that also.
But yes, I know what you mean, look at how much weight Mike Trout has put on since he was drafted. Is it legit? I really do hope so, but now there will always be that doubt every time someone puts on weight and has a good year. Or look at RB, the dude doesn't look like he is on anything, but his name is all over the place.