I've noticed that this varies in the game as well. It's kind of interesting because I don't always know what will happen. Some really gain weight while others don't. Active sims tend to not gain weight and I've had a few without that who have managed it even without working out. Most gain some weight and some gain a lot. If it really gets excessive, I have been known to go into CAS to slim them down a bit. Yes, I send them to gyms as well but sometimes I just go for lipo. I don't leave cakes in fridges when I rotate.
I'm not that strict about what they eat because I let them have variety. Most of them work out at least sometimes and some work out a lot.
For babies, the mothers do feed them a lot as infants but as toddlers I make sure that they only eat toddler foods not from adult plates. Even if a kid if a little chunky, they will usually outgrow it. I don't judge kids for weight at all.