If the Spurs wanted him to drop more weight, he would have. He's a PF at this point in his career. It's what we need from him now, with this roster composition.
That being said, I wish the Spurs would send him to Pete Newell's big man camp. Sean went while an NBA player and he said it did wonders for his overall game. Boris really needs to learn the finer points of rebounding position and post defense.

