In adults the eye does not change signicantly in size. However an apparent size change is relatively common and can occur due to the upper lid drooping which makes that eye look smaller and the other larger, or the eye socket swelling due to something like over-active thyroid, or wasting-atrophy of the fat in the socket such as with starvation or aging. Your best bet is to see an ophthalmologist and share your concerns and start ruling out the various causes.