As far as I'm aware of how it works and from what I experience, if any of your Sims reach a certain level in the Fitness skill (I think starting from Level 4?) They'll end up exercising (and esp jogging) on their own at times. It evens gets too excessive for me at times bc everyone starts to become a buff gymbro regardless of their traits (looking at you, Lazy sims who exercise all the time). Things get incrementally worse for their weight if you visit or live in San Myshuno too long. They'll pull any and all Sims (even poor vampires) to order food at he stalls so many times that they'll start packing more pounds then they can shave off, compounded even worse if they don't exercise much. But the situation changes all the time depending on what saves I go to and if I visit the gyms in game enough to see which Sims visit.