The pupils of both eyes are not the same size for a variety of reasons, the common ones are as follows: First, incomplete paralysis of the motoneural nerve. The parasympathetic nerve fibers in the motor nerve innervate the pupillary sphincter and maintain the pupil in a reduced state. Once the parasympathetic component of the motor nerve on one side becomes abnormal due to a viral infection or demyelination, the pupil of the affected eye will dilate to form a double pupil size discrepancy. Second, due to traumatic pupil dilation, when one eye is traumatized, the pupil sphincter is paralyzed, resulting in pupil dilation, which can also lead to different pupil sizes in both eyes. Third, misplaced medications, such as eye drops or dilating eye drops in one eye by mistake, can also lead to inconsistent pupil sizes in both eyes.