Sims getting too fat or too muscular in City Living?

My two Sims eat at the same time and generally do working out almost at the same time. They're very active and I do watch what they eat. One stays thin as a rail and the other either ends up chunky or on his way to becoming a body builder. It's super strange....and annoying...usually if they get too unfixable either by my having them eat nothing but salad or jogging I send them to the cas cheat code thing and fix the bodies myself.