My Sim was pretty thin coming out of CAS, but it seemed she gained weight rather rapidly. I bought her a treadmill and she uses it daily. Now it seems like my Sim has become extremely muscular, but ...
I ran into the same issue. I had tons of fun run on the treadmill repeatedly and she was back in shape. Only took one Sim day and I didn't notice any buffing up. Maybe only the lifting machine does that? Now I make her run every other day and she's yet to enlarge again.