It'll use it if it needs it. Toss in a bunch of 3D layers all casting shadows, plus multiple lights... and memory usage goes up like a rocket.
there's a difference between using and needing more RAM. Since you don't have much footage to be decoded into RAM, and most things are particles, only the CPU/GPU are used intensively on your render. To speed things up you could enable multiprocessing. For your config I'd give 4 cores to AE (leave 4 for other apps) and allocate 1.5GB to each core. As a bonus you should also see more RAM being used.
What the previous two people already said - you are primarily using a particle generator which only requires some number crunching to figure out motion vectors and such stuff, but consumes almost no RAM. If you would use more layers or other effects, that might be a different story, but in the current scenario it's perfectly normal...