So you are cherry picking a few people you knew, and one player, and saying that disproves the point? Young Reggie Miller or Tayshaun Prince are closer to our guys bulk than Barnes. Why didn't they fill out early too? Did they not put in the work? And how many of your marines were 7' tall? I'm willing to bet none. It's way, way, way harder to stack on weight and muscle in that type of frame. The amount of calories required alone is staggering, not even taking into account the ridiculous conditioning.
Barnes got older, his metabolism slowed, his body allowing him to pick up weight. And he's a professional who spends all of his time, when not beating up Derek Fisher, training. He's got access to the best nutrition and supplements (potentially drugs) on the planet. The best trainers, cooks, equipment, etc. It's a 24/7 job. It's not for these kids.
I'm not saying it's not possible, but certain guys have way more potential than others. Both Facey and Brimah have put on some weight since they got here; it's just they didn't start with much at all and therefore doesn't show. And unfortunately, running up and down the court 7 days a week for 6 months a year burns a lot off. Coupled with classes/studying instead of 24/7 training....a tall thin rail is at a real disadvantage.