Comment 27 for bug 24692

Usage of virtual address space is not the same as usage of physical pages. A process using up 100+ MB of virtual memory isn't actually a problem. For comparison, on my machine, a bit of mental arithmetic shows that the top few processes shown by System Monitor have over 8 GB of memory mapped, whereas the real usage is around 1.4GB.

If you are trying to measure the physical memory usage of processes, you should use the "Resident Memory" column instead, as far as I can tell.