@1g33kynerd Did your Sim age up at all unfit? I find it depends on the build of the Sim, but some look okay with extra weight, and some do look 'odd' (like their head/face doesn't look proportional to their body, etc.) However, I do find that if I open the Sim in CAS (Create a Sim) and either physically edit them a bit (you can push their shape in CAS) or I can re-dress them (choose clothing that compliments their build - like with us, certain styles just don't compliment certain frames/builds). For instance, IRL, I stay way away from yoga pants. They just don't work for my build. 😇 You can also edit other Sims (or their clothing) in CAS, if they're hurting your eyes. 🙌 I've definitely done that one!
If you like them to look 'optimal' in whatever type of clothing they or you want to choose, you can get them more fit (but if they bulk up with muscles you will likely want to modify which clothes will compliment them well, also). I don't like to deal with finding new clothes that look well on them as they change sizes, so I usually take a route I find to be simpler. I tend to add two (side by side) treadmills on my lot with a TV in front of it. This gives me the option of thinning them down by telling them to use the treadmill, plus I have them watch the cooking channel or something helpful. I can also send a second Sim to do the same thing and they will chat while doing it... increasing their fitness, cooking, social, and fun all at the same time. Then after they have eventually trimmed down they will usually autonomously go to the treadmill and do the same (though they will watch something more fun than cooking, on their own. Lol!)
Another trick that works fantastic – including for kids – is to use the ice cream machine (from the Cool Kitchen Stuff pack) to make 'Taste of Diet' ice cream and drag it to the refrigerator. After 2-3 servings of this ice cream your Sim should lose weight. (Got that tip from @PugLove888 ❤️!) Watch out they don't eat too much of it or they'll get downright skinny.
More tips on dealing with Sim weight can be found here.
Personally, The age change that frustrates me in ways I have no cure for is the jump from adult to elder. It's severe and they seem to jump from useful and active to decrepit. I'd much prefer to add some wrinkles and even clothing and other physical & personality changes to an adult rather than go the elder route for a Sim I'm actually playing. But I'm a rather 'Type A' player in Sims... and I have aging off, so they only age up if I have them do it. 😉