Is there a certain user field of view that's assumed for the parallax? For example, from my eyes, the screen covers a 30 degree viewing angle.
They must be assuming a larger viewing angle for movies versus at home. Viewing at any other angle is going to be out of focus somewhere, regardless if it's closer or further away. Hence, the eye strain.
Proper viewing angle is what you really want to achieve. Changing the display size forces you to different viewing distances to maintain the proper viewing angle. For 3-D this means everything to parallax correction.
I'm just thinking how an optical lens works. Same principles.