I just read your other thread about them going to the vet right before I answered this (poor little guy!)...I think it's because of the pressure on the optic nerve, specifically because there is a polyp involved. See the below link (you kinda have to read around the rest of the info, tho)...

cats eyes; two different sized pupils-symptom of peripheral vestibular disease? - Questions & Answers | VetInfo/QA

How is he doing?