Yep, its a limitation due to game architecture. The engine is old, developed by an indie company in 2005 when 64-bit and multi cores were either barely seen or only for the big boys (EA, BioWare, etc.). In fact the best I have ever seen it run is on a machine that is about 4 to 5 years old running with a 32 bit OS. So sadly you will need to limit how big things get.
