Sims gaining weight

You need to send them jogging. Sim foods have a variety of different "calories" (weight points?) assigned to them, but Sims never burn them other than with specific exercises - time, manual labor jobs, caring for kids, doing household chores and going up & down stairs, etc don't do anything.

Thanks!  I will try to get her to exercise.  Hope it is easier than trying to get me to do the same. Thanks so much for your help!

Some foods don't add to your sims weight, although this was not intended it was just never implemented.

This is mentioned in this bug report: https://answers.ea.com/t5/Bug-Reports/OPEN-Foods-that-don-t-add-to-Sims-weight/td-p/7674691

You can make your sim eat this food to prevent them gaining weight.

@irrilia Following are some of the things I've tried in addition to sending them jogging (I had trouble with my sims stopping when they wear out of shape and not sticking to jogging) and the things mention having to do with food...

Free Options (BaseGame)

• The treadmill is my go-to, as was already mentioned above by others... but I though I'd mention a couple of tricks I like to use. I place two treadmills side-by-side if I can. They socialize then and are more likely to enjoy the jog. I also go a step further if I have space... I put a tv on the wall in front of them. Then then I can tell them to watch a channel they learn from (like cooking), and at its best this method results in two sims jogging, learning cooking while losing weight, socializing, and having fun (due to the TV). 
• Have your Sim go to the gym and workout. There’s a gym in Willow Creek and one in Oasis Springs. Otherwise you could download one from the gallery or build one.
• If your Sim is energized you can click on the floor and direct them to do exercises. You can pause and queue up several.
• You can take the Sim into CAS and ‘push in’/re-shape areas of their body that have gotten fluffy.

Aspiration Points Reward Store Option

• There’s a drink called “Insta-Lean” in the Aspiration Points Reward Store.

Options That Require Purchased Items In-Game (BaseGame)

• Build a pool and have your Sims swim. This works for kids also, obviously.
• Purchase fitness equipment and tell them to use it. Fortunately, once you direct a Sim to use fitness equipment enough they will begin to do so autonomously sometimes. (Unless, perhaps, they are lazy or some similar trait.)

Options That Require Purchased Packs

• The Fitness Stuff Pack allows you to click on the TV and choose to have your Sim exercise.
• The Spa Day Game Pack allows your Sim to do yoga.
• The Cool Kitchen Stuff Pack includes the ice cream maker. With it you can make “Taste of Diet” ice cream. After your Sim eats 2-3 servings of this ice cream they will lose weight. (Tip courtesy of @PugLove888 )
• The Get to Work Expansion Pack allows your scientist Sim to create the “Slimify Serum”. (Tip courtesy of @cgrant659 )

More stuff...

• After they're fit enough they can choose a 'no sweat' jog! 
• I get my Sims the ‘Gym Rat’ reward trait asap so they don’t lose hygiene as fast when working out.
• The more energized your Sim is the faster the fitness skill will raise.

I have a harder time gettin children (and sometimes toddlers) to not be chubby, and sometimes resort to sending them to the ice cream (mentioned above) &/or swimming. It seems that if they age up chubby and I knock the weight down I don't see the issue return with the toddlers and children if they're eating and behaving in the 'normal' range of behaviors.

I often use the basketball hoop in the backyard for fitness and another player recently mentioned that Dream Big makes a Sim gain fitness skill rapidly. I tested it and it works. I can't find the thread again to thank the player that raised it. The hoop next to where the Spice Festival is held in the city works faster in my game, but adding them to a house or gym works too. 

@SheriGR I never knew that about the Taste of Diet ice cream I just thought it was low fat, what a great tip! I've had that pack for ages too. 🤦

They should add an option in the settings to disable certain features like weight changes

I'm not so happy that this happens to all the sims you play cause you get them to eat when they are hungry. One thing I do the slow down the weight gain is monitor how fast the food intake is and stop them eating when they are full or use U.I. Cheats and make their fullness go down so they can eat the entire amount of the meal. Exercise does help to keep the weight off but they do gain weight if you play on particular Sim a bunch. So then I use Cas.fulleditmode shift click the Sim in question and go into CAS and make them to the original proportions I gave them. I love The Sims but this is a little irritating that they gain weight so easily. Oh pool at home and swimming laps everyday helps too.

I just found this thread. I created a similar thread a while back, and I wish I knew how to merge threads (maybe only the moderators can do that though), because it feels like it would be good to have this topic collected in one thread, so that there’s a higher chance that this feedback is noticed by the devs.

Anyway, here’s my thread if you’re interested: https://answers.ea.com/t5/General-Discussion-Feedback/Feedback-The-Fat-and-Muscle-System-Needs-to-be-Better-Tuned/m-p/12239703#M69639