If you're seeing it in children, I've noticed when I put families into CAS through full edit mode while managing households I'm not controlling, that almost all children, even if both parents are still thin, are born about 2/3 to 3/4 of the way up the weight slider. I suspect the game reads pregnancy as the sim being overweight, and then there is a high chance the offspring will inherit that inflated weight.
In your active household, even eating two meals a day, especially if they are not salad or a yogurt from quick meals, your sim will gain weight over time if he or she doesn't also exercise. I had a lazy sim start off at about 25-30% up the weight slider as YA, and even eating half his meals as salad or yogurt, he was at slightly over mid-range when I later put him into CAS after turning adult. Because lazy sims get a bad moodlet after exercising, I didn't have him do any of it.
Previously, when I played said lazy sim and didn't watch what he ate, he had a steady diet of pancakes, BLTs, eggs and toast, and macaroni and cheese, with the occasional cake or pizza, and he was at about 75-80% on the weight slider around his adult birthday. It's gradual so at first I thought it was just a bit of bulking from tending the couple of garden plants and digging up nearby rocks for collectibles, but when I took a screenshot of the family at the table and I had a side view of him, I noticed he now had quite the gut. That was when I put the family into CAS and saw they had pretty much all gotten to that level. The wife had been pregnant a couple of times and just had a baby so to me it looked like she'd gotten a little bigger, but she was so much smaller than she was pregnant that it took a screen shot comparison to really notice how much weight she'd put on. I also think that on child sims, a bit overweight isn't quite as obvious.
I've found with a non-lazy sim, one quick meal and one more indulgent meal a day along with a jog or workout most days will keep them about the same. Sometimes you might have to do a couple days of salad only or longer workouts to even it out if there's a party with a lot of cake or something, but usually that's enough.