Depends on the Sims and their story. As an example Bobby Pancakes can stay the way he is (that might change in the future) and if his and Eliza's son becomes chubby I wont care. The female half of the Jang couple has actually gained a lot of pounds but I really like her that way. She is very pretty and looks quite happy in her skin. Don Lothario is one of the premades I don't play but boy has he become BUFFED! The man is huge! He is going to stay that way :lol:
On the other coin I have a former homeless single mother that was quite chubby. In her story she is trying to be successful as a singer and I had her tone down a bit. I don't plan on her to become rail thin but just a little lighter than she is now. At least two of my Sim ladies that got a pregnancy after-fat, hit the gym pretty ferociously. One half of my gay couple is hitting the gym too to gain some musclemass for preperation for the detective career and as a byproduct I see his other half at the gym as well (when I am not playing them) and he is getting a bit toned as well. As long as he wont start to look like Lothario or Arnold Schwarzenegger, I won't mind.
I have others too that will either stay how they are or make some changes in their lifestyles. The unplayed Sims however, both premades and homeless townies and npcs, I don't care about. They can blow up, bulk up or tone down by themselves. Their choices.