7 reasons why your pupils may be dilated

Focus
When focusing your eyes on a near object your pupils will constrict to increase the depth of focus in the eye by blocking the light scattered by the periphery of the cornea. Conversely, when you when you look at something further out into the distance, the diameter of your pupil will increase and muscles will relax in order to stretch the ligaments inside your eyes. This will pull on the lens, causing it to flatten and create a thinner contour allowing you to see distant objects in focus.