If you visit community lots with weight machines on them, gyms in particular, sims who are also visiting those lots certainly will gain muscle mass over time. This is very noticeable if you spend some time at Grimtooth Bar in Moonwood Mill, as the Wildfang hangout is very close by, and sims will autonomously go use the weight machines there quite a bit.
In a similar sense, if you spend a lot of time in areas of San Myshuno with food stalls, other sims will very often grab food from them, and you will notice those sims gaining weight over time.
It doesn't seem as though unplayed sims lose or gain weight/muscle mass while off camera, but their actions on camera have the same effects in those regards as with played sims. The effects are even more noticeable in neighborhoods where the Rock Your Body N.A.P. is in effect, as unplayed sims will autonomously perform all manner of exercise actions very frequently. (There is also a rather hilarious bug which will cause them to snorkel in the nude, if Rock Your Body is in effect at the Ohan'ali neighborhood of Sulani, and it doesn't have that effect in other neighborhoods.)
