He's not that type of a player. Youtube him, watch him in the Olympics, watch him anywhere but he's NOT a physical player on offence. Christ!
Plus, if he was to get heavier it would change his entire game and basically force him to relearn his natural position. He uses his size defensively and his long arms to his advantage. He should reminds people of a Shaun Livingston lite type of player in terms of his build up and a controlled Jason Williams on offence.