I'm starting to think it's a random mutation like that green brown hair that shows up sometimes, I had a sim born in my current dynasty who had one athletic muscular parent and one underweight parent and she was naturally obese, while her grandmother and great grandmother were plump they were nowhere near her size.
A parents weight during pregnancy can affect it as well, both mothers and fathers, I had a baby born while the father was also pregnant by aliens and rather large as men have no pregnancy morph they just get fatter and the child was overweight, that pair have had numerous kids over several games and never had an overweight one until then.
The overweight dynasty sims grandson was at almost zero fat, his mother was slim but not that slim and his father was athletic, no sense to it at all. As it was a challenge I used the body sculptor reward on them, but that is a temporary fix, if you have Pets plastic surgery is an option but expensive and I have no idea if those two work for kids. There is always testing cheats and adjusting the sliders in CAS if you really want to alter your child sim.
I have nothing against a sims weight
if it suits them, unfortunately for my dynasty two it didn't
