Running the retina iMac at "Best for display", as we know it generally acts like a 2560x1440 display. For apps that display photos, images are shown at actual pixel dimensions within the 5120x2880 display, so a 1920x1080 image, for example, takes up a relatively small part of the screen. The same doesn't seem to hold for video, however. A 1080p video takes up the same space that it does on a normal 27-inch iMac. When shown at full screen size, Quicktime Player reports "Current Size" as 2560x1440. Same when running VLC or iTunes. When I view a 4K 3840x2160 video, Quicktime Player says it's 1920x1080 half-size, and in full screen it's again 2560x1440. Anyone have an explanation for this?