This normal variation is usually not noticeable from casual observation. There are certain pathologies that affect pupil size, however, and if you notice a sudden, significant difference in the two eyes you should consult your eye doctor. In the dark, the small muscle that dilates the eye contracts and the muscle that constricts the pupil relaxes, resulting in a larger pupil. This allows us to see better in the dark. The opposite occurs in bright light so as to restrict the amount of light entering the eye.
Pupillary diameter decreases with fatigue, and sleep is accompanied by even smaller pupils.
Arousal induced by fear, interest, pain, or excitement causes pupillary dilation. The reason for this is unclear.