@numbuh4 --- Skill level 1 used to work, so, you could keep trying to see if that gets fixed. Even if they may retain a higher fitness level than you prefer, if you want to change your Sims' appearance, then use the following cheat ---
after testingcheats is on
cas.fulleditmode
Then, press esc on the cheat console (box)
click on the Sim, and click, modify in cas.
You may change anything about your Sim, including weight and muscle tone in cas --- with that cheat enabed.