I'm a big girl and I make all my sims skinny too. Funny, but I live my life through them, for example, I'm very shy and quiet in real life, in the sims I'm very outgoing. Go figure lol. Back on topic though, I've found if my sims are chubby, and I use the body sculptor on them right before trying for baby, the baby is always skinny. If I want to change hair, I always do mirror, body sculptor, relax. It works 100% on the weight, but it's a 50/50 chance on the hair.