The only issue is that in my testing the size value has no impact on the 3d objects
I disagree as I did some testing with the Miami City Skyline, which is one massive tmb object that's a 10-mile long string of buildings. The DLC has SIZE=50000 (~31 miles) for the Miami Skyline TSC file. If you completely exit out of AF2, and start from a location outside of this 31 mile radius, you won't see the downtown skyline until you fly inside this radius.
So IPACS gets you into the cockpit for that first flight as quick as possible by 1) only loading the nearby TSC files into memory, and 2) keeping the SIZE values small. However, to avoid major landmarks from "popping" into view, they increase the SIZE value significantly.
If you change locations several times during the same session, I believe those additional TSC files are loaded / added into memory and the previous TSC files remain in memory. I'm not sure what happens once your memory is full - I'm guessing some of the data gets cached into storage (SSD / HDD).