I went through a period in my game where nearly all my sims were blimping out after eating a single meal. I ended up figuring it was a glitch, because there's a difference between a sim kegging out for a full week and then gaining weight, and a sim gaining weight from morning to afternoon after eating a single meal.
In regards to my Sims now, some are obsessive about exercise (I'm looking at you, Grim Reaper), others are couch potatoes unless I direct them to do something, and the rest are somewhere in between.