Maybe he's wondering about training, when you create a player who is only like 16 sometimes they grow up to like 8 inches during the dynasty, i think you must be really young for that to happen though
There's no way to force it as far as I can tell (aside from manually making a change through editing appearance.dbf). Players with a good work ethic will stay in shape, get in shape and add more muscle more often than not but players growing another inch or two seems to be fairly random amongst young players.