If one renders a Image of 1500 x 1500 he will expect it to be that size and not 10 times larger because of increasing the Pixel per Inch for better detailed quality on the mentioned size. If you render screen resolution ( Size ) and increase the Dpi you will get a higher detail on the actual size being able to zoom in 200, 300, 400 % on small details without a loss or blurry artefacts.