My sim also looks like a body builder. She became pregnant and she was huge! during the third trimester of pregnancy. After she had the baby she still was quite chubby.
To make her lose weight I let her run around the neighbourhood. That seemed to do the trick. But after she was thin again (BTW, she is also a gardner) she started to get some serious muscles. I was thinking it was because of the running, but now I also think it may have something to do with the gardening.
I am trying to trim her down again, so I stopped running. But she still tends the garden. It has been about 4 simday to a week and she is still as muscular as in the beginning. So I'm gonna try that potion soon, or the cheats. I'd rather not use any cheats, but I guess I have to if I want her to look normal again
the other sims in my household (husband and kid) look fine. they don't gain weight or anything.
My sim started to have problems with her body shape after her pregancy. No idea if it has something to do with it, but who knows. It's all a bit strange to me.