Well, this is related to the single core problem and not the memory... you can have a huge multistar map with 10 player, along with the sins optimization mod... the game after a few hours will slow down and ram used can be lower that 800 mb !!!
With sins, you are better with a single core at 5 ghz that with a eight core 2 ghz...
Yes, the Buffs system and ship movement and AI workload, while working great on short small games, becomes a mess on huge maps and/or long games. And as with other Strategy games facing similar problems, the more you build, the slower it gets.
If they only used 1 thread per AI and implemented a pathtracing system that is thread-aware...