scolaner :
Because the rendered image is so small (just as it is on the HoloLens), it requires little in the way of compute resources, which enables lightweight and slim eyeglass-type designs.
Unless you're talking about Hololens' small FoV, the physical size of the projector has no direct impact on the compute resources required. It still has an effective resolution (1268x720 per eye), and that image resolution still takes as much horsepower to render - no matter whether it's being displayed via "light engine" or LCD screen.
The story behind Hololens' weak GPU requirements really comes down to the low resolution & quality of graphics you get - more like those of cell phone-based VR than what you see in Rift and Vive games.
MS clearly carried over this experience with lightweight GPUs to enable their PC VR HMDs to have baseline requirements satisfied by typical laptops.
BTW, light field projectors still remain the gold standard in AR display tech. I'm really hoping Magic Leap gets you guys a demo unit, soon. Should be sometime this year, we're lead to believe.