Think of it like a picture 20x40 pixels can't make out what that looks like, 40x80 is now a tall icon and you can see a little more, once you get to a certain rez you can see the full image. Anything more will just be more clear/sharpness to the image. Fluid solve is kinda like that.
Now think about painting an image in photoshop. if you have 20x40 pixels how you paint that image and how you exaggerate colors and contrast to get it to show up and read is quite different then how you would paint it if you had more resolution.
This example is just to say low rez doubled is noticeably more readable where higher rez ends up just having more detail.
(click to enlarge)