* Horner's Syndrome: This condition affects the nerves that control the eye and can cause a smaller pupil, droopy eyelid, and sunken eye.
* Iritis: Inflammation of the iris (the colored part of the eye) can cause a variety of symptoms including a small pupil.
* Glaucoma: Increased pressure inside the eye can cause a dilated pupil.
It's important to note that I am not a medical professional, and this information should not replace a veterinarian's diagnosis.