Down-in-Simsland I noticed this when I was looking through S4S for the "litter" issue and wondered what it was. Thanks for clarifying.
Different, but similar I also notice a lot of photo and paint from reference images that I knew where long gone from my Save. Seems like there are a lot of issues where the game has no method of cleaning out cruft.
All of these probably add to the bloat.
Yes, any texture that is added to the save, like photographs or painting for reference are there forever, even if you delete the associated item.
If you use custom paintings from the gallery and add a full wall of those paintings on a lot, only keep one of them, and delete the rest, all the pictures will be added to your save file.
