I've encountered this before, but now I've got screenshots to share. I created heavy (tubby) Sims for my Maximum Moodlets challenge (Arthur and Elaine). They stayed heavy throughout the challenge because I just liked them that way. But once the challenge was over and I continued playing them with their two daughters, I had them start working out when the daughters began growing up. Both daughters were tubby through their childhood years and into their teenage years. In fact, they look so much like their mother that I wonder of Arthur was involved in their conception at all. Hehe. The teenage girls started working out with their parents. They trimmed down in an astonishingly short period of time. One workout is all it took, in fact. They weren't the heaviest that the game allows, but they were definitely tubby. And after one workout on the treadmill, they were normal weight. They started gaining it back right away and were back to their original size after less than two days. Another workout would bring it back off. If they work out every day or every other day, they will stay trim. When Arthur started working out, I put him on the weight machine first. I saw no noticeable difference in his size. He might have been a bit more bulked up, but he was muscular to begin with, so I couldn't tell for sure. The next time, he got on the treadmill. And sure enough, he trimmed down just like the girls had. Now that they have some athletic skill, they have interest in working out, so they will probably stay normal sized.
Elaine got a wish to have another baby with Arthur and I made sure that both of them were in their trim states when they tried for a baby. I want to see if this child will take on the tubby trait like the older sisters, of if the new baby will take on their trim trait at the time of conception. It's a little interesting, isn't it?
As a result of this discovery, the family has a home gym and has installed a pool. One of the daughters had gotten back to tubby weight and I put her on the treadmill. I've got screenshots of that workout where you can see how she's getting trimmer during her exercise and how at the end of it, she's very much smaller than when she started. And it's all in ONE workout session. Lol. Check out her pictures.
Here she is as the start of the workout. This is her normal weight from birth.
Midway through the workout, it becomes noticable that she's getting smaller.
Here she is at the end of the workout. She is completely trim and not the least bit tubby.
Here's a full front shot just before she got on the treadmill. Look how much difference only one workout makes.
Seriously... why can't it be this easy for real people?? Haha!