Well, he's only 20. So it's really only natural that he starts to fill out muscle wise as he hits his early 20's (btw, I'm 20 and actually am starting to see quicker results in weight lifting than I did as a teenager. Strange, but makes sense)
The guy works hard and is a big sponsor for a protein supplement company.
But, yes, this is very scary as a Mets fan lol.
Also, about his speed, I don't see him dropping too much off. His speed is based on his hustle and technique, not because he was skinny. I mean, jeez, have you watched this guy run out a groundout?! It's scary how much he hustles.