I dont know of many portable regular dvd players that have screen sizes as big as most laptops with HD capability. So the difference would be less
Also have to keep in mind that some of the difference you are seeing in your computers is the fact that the graphics are different sizes - they take up more of the screen at a lower resolution, which makes the difference much more apparent. With a movie, the same picture is on the screen either way, so it comes down to whether or not you can make out the difference in pixel count with the same picture.