I had a child sim - who completed the motor skills aspiration, btw - and it did not seem to matter how much I had her exercise, she was still noticeably thicker than other sim kids. I didn't micromanage their portion control, but I didn't have them eating when they weren't hungry either (there was a period in time when there were desserts in the fridge and I noticed that if desserts are available and you tell a kid to solve hunger themselves... they will eat all your flirty cookies. So that might have been when she packed on the pounds). Whatever, nothing seemed to take them off. I was having her take brisk showers so that she could do sit-ups, she was spending her life on the monkey bars... nada. So just before her birthday, I spent the points on a lean potion. Skinny kid... has birthday... turns into chubby, like very chubby, teen. Spend the points again, chubby teen drinks another lean potion and stays chubby.
Her parents were problems too, but differently. I noticed her mom getting a little thick waisted and took her to the gym to spend some time on the treadmill (much more effective than going for a jog, and a great way to build friends) and she slimmed right down to her playing weight after just one trip. Dad was another story. He was athletic, and right up until he became an elder he was movie-star buff. As soon as he hit elder status though, he started getting exhausted moodlets from his daily jog and the weight just started piling on. Eventually I discovered that he can still handle the endurance mode on a treadmill and it's almost all off now, but he was getting huge.
Meanwhile, I now have the second adult female sim (first was dad's sister, now another fem in a new game) who puts on muscle for no discernable reason. This second fem isn't athletic, she's a creative bookworm perfectionist writer. The muscle just appears, and it's getting freakish.
