They promoted him too fast to A ball, before he was physically ready.
Two years later, he's probably filled out a bit.
These sort of players reveal themselves, if they're truly top prospects, after a period of struggle (swing adjustment, learning to recognize breaking stuff, etc.) they take off, dominate leagues, make multiple jumps and arrive in a couple years.
if not, then they're exposed as more run of the mill talents who may develop after 3-4 years of gradually progressing up the ladder, but are unlikely to be ML stars.