Harper was listed as 6'3" 205 when he was a 16-year old in Sports Illustrated, and now he's a 6'3" 225, fully developed hitter. There's no reason to think he's going to magically grow 3 inches, and he doesn't look physically immature, like someone like A-Rod did when he first came up (6'3" 190).
When you're 6'3" 225 lbs, you're not filling out anymore, you're just bulking up or getting fat. There's no reason for Bryce Harper to do either, if he doesn't want to.
This is like some Boswell argument using the analogy of Adam Dunn without ever admitting that Adam Dunn is a big ol' fat guy now.