Brightness and Contrast
The amount of light on an object determines how you see it. A brightly lit object appears larger than a dimly light object. The basic idea here is that everything looks larger if it reflects more light. This causes an illusion. For example, a brightly color box of corn flakes would appear larger than a darkly colored box even if they are actually the same size.
The background that an object sits on also has an affect on the way you see it. If the contrast between a bright and dim object is too great, you will not be able to see the dim object. For example, you cannot see a car behind a highlight that is shining in your eye at night. For another example, a gray letter on a white background will look bigger than if it was on a black background.
Sometimes because of the brightness and contrast of an illusion, your retina gets tired. This can cause you to see an afterimage. To learn more about this, click here.