Do you feel Stones can put on muscle mass whilst maintaining his current athleticism? I remember Kompany doing it around the same age as Stones so I may hold out on him until he beefs up but one difference Kompany had was desire and aggression. Komps always had that regardless of size whilst Stones doesn't. Gone on a tangent a little but intrigued to see how Stones will progress from here.
Stones doesn't lack physical strength, but aggression. He's not huge but he's big enough that if he wanted to put himself about he'd get the better of most forwards physically.
Unfortunately a lack of aggression is his biggest problem right now, and it ties into how much he's backing off players instead of taking them on.