Yes, you cannot simply say, "oh, lets use less CPU power and utilize the GFX card more". While they do have a relation, you must imagine them as completely different entities. The CPU is doing the physics, while the GFX card is handling the viewable representation of what the CPU calculated. As the graphics effects are pretty simple, the GFX card has a relatively low workload. But to reduce CPU load, you'd have to dumb down the physics calculations, making them less accurate.