Sometimes, this can be happen due to an injury to one eye. If the eye get damages, the eye muscles can control the aperture of the pupil. But it doesn't impact our vision too much. And the pupil can recover as the time. Also, light sensitivity can also cause one one eye more dilated than the other if one of your eyes are more sensitive to light. However, if you noticed that one pupil is very obvious bigger than the other, you'd better consult a doctor for that may caused by some serious problems.