The pupils in the eyes constrict, or become smaller, for a variety of reasons. These reasons include: a person is confused or overloaded with information, from the effects of certain drugs, and bright light.
Eye pupils normally become small due to exposure to bright light. In this kind of light, the pupil contracts because light creates a high impulse causing muscles to close the pupil. The size of the pupil can reflect the body's sate and mind so a small pupil can illustrate anger.