Keep in mind that what matters in the end are not the pixel dimensions (i.e. width and height of the video in pixels), but the actual resolution of the source material. You can blow up a standard definition video to 1920x1080, but that doesn't magically increase the resolution.
In my experience the quality of iTunes 1080p videos is rather low when compared e.g. to Blu-ray (which is not surprising, since iTunes HD files are far more compressed). This is really obvious on big screens. I doubt the resolution after compression is anywhere near "real" 1080p, much less even higher resolutions. Combined with the small screen size, there is a point of diminishing returns ...