With some not always true assumptions here... sometimes the Fullscreen is useful for sampling raw images or parts since it's of a higher resolution. If you think about it, you are cutting of the sides of the picture (which is terrible!) but you are then using the full resolution for appx half of the picture. So the half that is there is actually of a higher resoultion than it would be if the other half was also there.

Unless the fullscreen or zoomed image was taken from a pre-existing widescreen image and therefore can contain no new original pixel information- rather interpolated pixel information from the original widescreen master.