Stress can cause dilated pupils if it stimulates the sympathetic branch of the autonomic nervous system, which triggers a fight or flight response, according to Scientific American. Stress can also inhibit the parasympathetic system, which triggers the rest and digest functions, and cause dilation.
Emotional events such as stress cause smaller pupil responses than the light reflex, notes Scientific American. Generally, cognitive or emotional events cause pupils to dilate by less than half a millimeter. At any time, pupil sizes reflect the balance of the parasympathetic and the autonomic system, as well as the mental effort exerted into current tasks.