So I just popped into my game and took some 'after' shots of the Sims I hadn't yet set back to their 'standard' weights.
This is what I'm talking about...(sorry screenshots aren't the best)
Makaria Reaper at her 'standard' weight.
https://i.imgur.com/s4CxnLe.png
Makaria Reaper 3 Sim days later when household not being actively played.
https://i.imgur.com/QNgJMPn.png
Selene Reaper at her 'standard' weight.
https://i.imgur.com/3gOX4XX.png
https://i.imgur.com/1K8KMCq.png
Selene Reaper 3 Sim days later when household not being actively played.
I've had sims gain weight in a more natural progression before, it's something I factor into my headcanon/gameplay. One sim I had was a foodie and a chef, so yeah he gradually stacked on a few pounds and had a bit of a paunch and thick waistline - not an issue, it made sense to me that this sim would gain weight due to his trait, the fact that he worked around food, and the fact that he worked nights so was more likely to grab unhealthy takeaways on the job (my headcanon/gameplay story at least).
3 days serious weight gain though is just very odd to me, from a story progression point of view especially it makes little to no sense (to me at least). I actually don't mind if Grim's kids do put on a bit of weight, because they tend to be a bit too on the thin side sometimes as it is - but not like double size in that quick an amount of sims time. Like, WTH are they eating when I'm not actively playing them? :D