If you don't want them gaining wait, you need to have them only eat salad and go jogging frequently. The calories system is annoying in Sims - doesn't matter how often they run up and down the stairs doing laundry, hike through the jungle looking for antiques, chase toddlers around the house, skip meals... the ONLY way to burn calories is to use some of the official exercise interactions, even though most foods add them.
@PipMenace You bring up some great points. Thank you for sharing what works well for you! It's really helpful to folks who are new to the game to hear ideas of methods of dealing with various aspects of the game that can be tricky, annoying, difficult... or that can snowball into something (like this area). 🙌
Many people do what you mentioned – only have them eat salads (and low calorie foods) and avoid desserts – and find it works very well for them. @1g33kynerd, if you need this, there's a great 'Carl's Guide' with tips on food/weight gain, etc... and that has a list of the calories in foods in the game: https://www.carls-sims-4-guide.com/tutorials/calories.php.
Personally, I like having a couple of Sims on the lot who do most of the cooking and baking (or I have a butler do it) – and I just direct these to prepare a meal a couple of times a day, or sometimes two meals in a row, and I drag it/them to the refrigerator. Plus I rarely make desserts. This method has worked well for me in that they autonomously grab a leftovers when they'e hungry, and they don't seem to overeat, as they aren't combining this with me calling them to a meal when they're not actually hungry yet. However, if they get at all in the red they may be 'out of their mind' a bit and not do it autonomously, so then I will direct them to 'grab leftovers'. Anyway, this system has worked for me, but there are a number of ways people deal with this in-game that are very creative and work well (somewhat like the parallels in real life).
I used to send my Sims out to jog, but then I began finding that some of them would not keep running. I guess they weren't in the mood. 😉 So I went with on-lot options so that I could keep my eye on them. It was interesting that after they began using the treadmill regularly, especially with the TV, they seemed to get in the habit of it and not dislike it... and would do it autonomously. This was before the current system where they decide on their likes and dislikes and you 'approve' that, but in some cases you can hope they may be willing to add physical fitness as an interest/like they are 'into'.
