This problem also exists in Windows and the Live Branch, but I did all my recent testing in Linux. Starting with this save -- 5cb3cc6f3d94c_TheDarkestCitadel.sav -- that was linked in this thread:
I ran several tests. After running for close to 100 cycles, the game's memory footprint increased by about 700mb. I then loaded a mod ( https://steamcommunity.com/sharedfiles/filedetails/?id=1706146115 ) that disabled the auto-save feature and ran for 20 cycles with negligible increase to the memory footprint. I also saved the game multiple times in a row and saw the same memory footprint increase as seen while using auto-save. There was also a minor FPS performance hit following each autosave that, over 60 cycles or so added up to about 5fps. Quitting to the desktop and reloading the game at that point removed the performance hit. This drop in FPS was not observed while running with the mod that disables autosave.
Additionally, if you load the game, quit to the main menu, then load it again, each time you load you will increase the game's memory footprint by 400mb or so. Quitting to the main menu does not appear to release the map from memory.
Steps to Reproduce
Run a high-cycle game for multiple cycles and observe the memory footprint growth. Reload from the desktop and notice that memory is now free. Run the same number of cycles with auto-save disabled and observe the is no memory growth.
This issue has not been confirmed by a developer yet.