My game still leans towards everyone putting on weight all the time. I like to have a variety of body types and I love the variety of food in the game so they never diet. Some just get large and stay large and others get large and try to work it off and others are so active from a young age or start with such a small body type that they never get large no matter what I feed them.
Mostly I use yoga or the treadmill or sometimes the tv workout for middle aged sims who want their younger body back. All of the other equipment is used by sims who want muscle, which usually starts in the YA phase.