Because I don't like to use the money cheat and I love skilled up sims, I built a "Skilling Mall/Lot." It is technically classified as a gym but it has an inspired section (with instruments and easels), a focused section (with chess, computers, crafting tables, telescope, microscope, etc), and other large items that a lot of lots don't have room for (like a playground, pool, and rocketship). Since it is a gym according to the game, my sims have gotten super buff and the only overweight sims I've seen recently are the newer EP NPCs.
For a while there (aging on or off doesn't matter), my foodie and non-active sims were gaining quite a bit of weight.
If you're wondering, I think the game is designed to work like "real life" caloric intake. If you sims keep eating a meal (because they usually finish a plate) after their hunger bar is full, that is how they gain weight. If they never exercise, they get bigger and bigger if they keep doing it. If you make them stop eating when their bar is full, then they maintain their weight.
So don't feed them lobster thermidor if their hunger bar is less than halfway, do a smaller meal or stop them when done. Certain meals fill the bar faster than others. I've been known to stick the fridge in the household inventory to keep sims from eating before their hunger bar demands it when I have a chef in the house who makes lot of rich food.
To avoid super muscular sims, stay away from boxing and weight machines. I think treadmills are mostly okay.
Sims also have their natural body type (as generated or created in CAS), which can be super thin to rather wide and this shape is what their "fit" self will go toward. I have some rather husky buff sims and some "looks like they never ate" buff sims (which is rather funny looking)... Most of the time the sims are getting fit when I'm not playing them--they are just going to the gym while my household skills up.
You can choose another lot type (like a park) to avoid the buff sims or make sure their aren't weight machines or boxing items.
