I think it's because the fats rioted (as usual).  I believe the start of that episode had fats with pitchforks and torches circling them. And their little experiment with running track had one fat guy, but not a ham planet.  Let's see someone like Boogie complete any physical activity.  (BMI as a health measure might be inaccurate for extremely muscular people like the The Rock, but for average low-activity people it's perfectly fine.)