They've actually nerfed fat gain since Sims 4 released, when you compare obesity levels in the western world our Sims have a relatively easy time of staying in shape. One of my Sims is a glutton, but also active. She's an Astronaut and the house contains a treadmill, she autonomously pounds the living daylights out of the thing night after night and her husband works nonstop in the kitchen making cakes so she doens't waste away.
I did a "fat camp" challenge recently where I took 8 fat, lazy, slobbish, glutton sims to see how long it would take to slim them down, controlling only one sim at a time (Switching control at midnight). Using "encourage to workout" interactions, got the whole household slim after about a week. Once you hit level 4 and can "endurance run", game over. Usually there's an abrupt change in size the moment they hit Fitness level 5, instantly transforming from "obese" to "curvy" or "slim".
On my normal games, I always put a treadmill in the living room in front of the TV or next to the stereo. In fact I don't bother much with dining tables and sofas are a low priority. The living room is a space where my sims can work out, paint, write, or program while getting entertained by tv and music and still be close enough to socialize with one another. It's also important to have a nice bathtub so they can soak off any sore muscles post workout.