That's interesting. I'm curious to know what bloats the save files as well. If I ever have performance issues, it would be a good experiment to tone down those areas to see if it makes a difference.
I play on a main save that I've had going for years so any info helps. I suspect that also having cluttered lots probably contributes to that as well. (Don't know for sure.) Especially for those who play rotationally and build a lot of them. I'm guilty of downloading many cluttered Gallery lots because they're so awesome and I fill my towns with them. I'll have to take a peek at my save file some time to see what the damage is, lol.