rpict memory usage keeps increasing with -aa 0

Radiance whizzes,

I've upgraded to 8 GB of RAM, so, naturally, I found a scene that needs all that space. The rpict is eats up 4.6 GB initially, but even with "-aa 0" (turns off ambient caching, right?) I am still seeing my VIRT and RES memory sizes growing. They are at 5.3 GB after only 10 minutes of rendering. I am worried that I will not be able to render high-resolution images of this scene now. How can I prevent this?

FYI: the scene is composed of a sky and 407 "instance"s of separately-compiled octrees, each with 20k-100k primitives.