It's becoming common wisdom that JJJ needs to add some weight to his lanky body. I wonder why. He's plenty strong enough right now (per reports from training camp and observing him in the post). And the theory that you need bulk to compete with the tall trees in the game doesn't hold a whole lot of water once you consider that some of the best players in history have been around Johnson's size. Off the top of my head, there's Bill Russell (6'10" 220lbs) and Kareem Abdul-Jabbar (7'2" 230lbs). Johnson is listed at 6'10" 221lbs.
Hell, if Tayshaun Prince (listed at 6'9" 215lbs, but sure looks less than that) can succeed in this league all these years, then JJJ surely can. Prince looks like a skeleton next to JaJuan. (Well... next to anyone, I guess.)
I think each player has an optimal weight point for the work he has to do. It's entirely possible, even probable, that Johnson is at his.
JJJ's body most closely resembles KG's, and nobody ever told KG to gain weight (and if they did, they were mistaken). I'd leave JaJuan as he is. If he bulks up naturally a bit, over the years, then so much the better so long as it benefits his game.
Meanwhile, the kid's looking like he has tremendous potential. Will he grow into it in time to help us this season? Here's hoping. (Imagine having 2 KGs. Yeah, that works for me!)