Definitely can gain weight that way. I had a family that ended up with three having birthdays in the same week, so had one party and invited all that could be invited. Set up a buffet table and none of the guests ate from it, but one of my sims, she couldn't seem to stay away from it. I had some of the family members take the food to the fridge to save and instead of taking them to the fridge, she was going outside to eat.
Noticed later that she looked different, sent her inside and she was huge! It was Bella Bachelor, so she wasn't the least bit overweight before the party.
Fortunately, I sent her to bed and for the next three days had her working out a lot. Didn't let her eat until she was really hungry.
She lost all of the weight and returned to her normal size and stayed that way. I had been worried that she would return to the overweight size after she lost it all, but she stayed nice and slim.
Anyhow, they can gain weight very fast in one night, but can lose it and keep it off it seems. Except for the male sim who become pregnant when I had SEA, he had the alien baby and never returned to the nice sexy body. He remained quite chubby.