I have a default project that I start from each time. It has many external instruments defined, with a dedicated track for each. There are no other tracks defined. I can open this project and let it sit for several hours and it uses up all of the system memory. I can sit there and watch it slowly increase, even though nothing is happening, the project is idle.
I turned off the “MIDI Retrospective Record” option after seeing it mentioned as a potential problem. I never had this problem until I defined all of the external instruments.