The last two kids born in my game had parents that were max fitness, low fat. They had two daughters who ended up with maximum body weight when they aged to child. They weren't eating unhealthy food, the weight slider seemed to reach max when they aged up to child. It sat in the middle when they were toddlers. This used to happen with just girls but I haven't had any boys born in my households lately to know if it's still happening to girls or to both. I ended up going into fulledit mode and set the kids' sliders to sit in the middle and they aged up to teens at a normal/average weight.