Jinora's penchant for spirituality made me overthink it. I think that if you're there in spirit, you don't gain weight since you're there in an ethereal form, but if you go there physically, you still have a body and the rules of the physical world apply. I don't know, but someone on another board pointed this out. They said it looked like he was trapped in the same tree that Koh lives under. A prediction... If Raava does in deed have to rejoin with Vaatu, that leaves Korra without an avatar spirit. So what of the avatar? Maybe Korra's spirit on its own will be strong enough to carry the four elements and continue to be the avatar. The training wheels will come off after 10,000 years, so to speak.