I hate to say it, but your chubby girl looks just like my chubby girl. My girl Mimi has been gradually getting fatter and fatter. She has a normal sized frame, but the fat is all belly. She has normal alertness, and waddles around, sleeps with everyone else, etc. She takes all the blocks and hides them away. She gets treated the same as all the other ratties, but she is the only obese one. Go figure. My girl seems normal except for her weight. Maybe rats are like people - some are prone to obesity.