You're absolutely right, I have same observation as you. It's not water retention. My bet is little bit in her thighs. They look full of fat when she sits but standing up not as much. And as for her belly, I don't think it's hanging lower it's just getting massive.
I'm impressed by her latest weight in. 13 pounds in a month, and at this rythm she is going to get beyound 570 by december if she doesn't push it. The question is: Has she lost control or she's gazing like a cow to keep her weight and plus to be able to grow?