I noticed some things. Holocake 2 isn't a lot thinner than Cambria. In fact, it doesn't have processing, so adding that in could make it just as thick. The thing about holographic optics however is that while emulating the pancake optics, it does it in a fraction of the weight, since they are essentially just thin plastic films. Compared to thick pancake lenses, that's a huge reduction.
Notice how Holocake 2 doesn't have a top strap, and there's no halo either. And yet Norm still said it was extremely light and comfortable. That is thanks to the holographic optics. Perhaps you might be able to imagine this headset as almost hollow feeling.