I think you are onto something there, both with your formerly bloated saves (it's the individual nhd files within a save that should be checked for outrageous file size by the way, but 3.6 GB sounds huge for one individual game save no matter how it's measured) and with routing issues in your currently being played world potentially being heavy contributors. Very heavily populated worlds will also do this, especially if any form of story progression is running as it tries to manage what could be far too many sims all at once. MasterController can be used to get a quick reading of the resident population, on City Hall NRaas > MC > Demographics > Population > "X" to dismiss the filter. It's the Resident number that would be important there, not Service or Homeless unless you for some reason have those in the high hundreds or thousands.
What can also help control resource usage are playing offline (meaing not logged in at the Launcher or in-game levels), disallow the in-game TS3 store shopping experience, disallow the hidden object startup game, switch off scrapbook memories or use a mod to control them.
The more or less complete list of NRaas mods that are designed to improve things in this regard would be Overwatch, ErrorTrap, Register, Traffic, to an extent GoHere, Traveler (if any form of world travel is invoked), MasterController (for an occasional Reset Everything run from City Hall). I can't imagine playing long-running ongoing games without these in play to keep things moving along smoothly.
In case you are not familiar with this page, here is a more complete set of tips to help improve game performance and resource usage.
https://www.nraas.net/community/TIPS-FOR-BETTER-GAME-PERFORMANCE
