Actually, in the wild the snakes don't typically reach their max sizes like they do in captivity. Sooo, in a way they are bigger in captivity, technically speaking. But the numbers given for the species take that into consideration and are based off of what they are capable of physically getting to.

Burms ~20 is the main one I know, but males are supposed to run a little smaller to max at around 16 on average. (Mine is creeping up on 7'. :] <3)