The size of the monitor doesn't matter, it's the resolution. at 1920x1080 there are exactly the same number of pixels on a 20 in display as on a 100 inch display. It takes the same processing power to generate the picture on either display. Now the closer you get to the display, the pixels would appear larger on the 4o inch display, and therefore not as sharp as on the 20 inch one, but your graphics card doesn't care about the size, it is still having to generate 1920x1080 pixels.