A cat’s pupils are controlled by the optic nerves and the brain, which sense the amount of light entering the eye and adjust the size of the pupil accordingly. When the optic nerve is damaged or under pressure from an infection (or in the worse case scenario a tumor) then the eye will not respond to these stimuli in the same way.

The fact that only one eye is affected suggests that it is not a dietary problem. The most common cause is glaucoma (build up of fluid in the eye) and will normally be accompanied by redness, teariness and/or cloudiness of the eye. You need to take your can to the vet to have a thorough eye exam and to test the pressure in the eye. Your cats vision is a risk if you do not treat this condition as soon as possible.