Every bit of food in the game has a set of calories. I remember ages ago there was a mod that actually showed those calories but I can't remember it. But lack of exercise, childbirth, and simply eating when not necessarily hungry (fully green) all play factors for adults.
The toddler/child weight is determined by three things in this factor:
1. Parent's gene weight (fat scale)
2. How much it was fed as an infant.
3. Lack of exercise (overtime)
So if your toddler has rather thin-ish parents but is a chubby toddler, It's possible that your sims could have used the feed interaction a little too much while it wasn't hungry as an infant.
Edit: forgot to add the 3rd lol.
