My runtime is about 398GB (1.4M files, 53.7K folders), so I don't think runtime size matters.

This is a intel quad system with 8GB of memory running Vista64. I get similar results on an i7 with 6GB on Windows 7-64. On this last system I have the external runtime attached as a networked drive. So it is a bit slower when filling the library.

What tool do you use to monitor RAM usage? I run (Microsoft's System Internal) Process Explorer all the time which gives me a complete overview of what every process and program uses for resources.

Memory usage is very complicated issue, especially where virtual memory is concerned. In task manager you can look at the "working set". That is the actual amount of memory the program has allocated and reserved for its own usage. This will increase when you load figures and props. Once you reach the 4GB barrier on a 64bit system you run out of memory on Poser.