Originally Posted by
James Metcalfe makes sense, can you tell me what these other factors might be?
Sure. While watching a movie on your phone/tablet and the video is full screen (it usually is) the video is rendered using the graphics chip frame buffer directly so that no cpu based memory copies are used. Something like that won't really happen because in VR you're rendering the movie on a GL texture in a 3D scene where every polygon is being computed for each frame (60 per second if your device can handle it). That alone should consume much more than normally.
Also many different sensors are constantly providing data, like the gyroscope. This drains the battery a lot (although as far as I know there are dedicated chips that can handle such tasks without bothering the cpu much on some devices). Same applies to the accelerometer, compass etc. Surely there are other things to consider specifically in VR, but these are the main ones I can think of at the moment.