In some games you select mobile and then it "lowers the graphics" in some way but how do they do it?
Well you see, if there is an image that is 300x300 in size, and they want the quality to go down by twice as much, they'll shrink it to be 150x150. How they do that (presumbly), is they take one-forth of the pixels from the old image, and use that to build a new one.