The adult eye does not change size as the eyeball wall sclera/cornea. Are extremely strong and rigid. However the eye may appear to be larger or smaller due to several problems, some of them very serious, that may make an eye look larger or smaller.
The first of these is if the eyelids, mainly the upper lid is lower on one side than the other (ptosis) or elevated (eyelid retraction). The second is if the eyeball is being pushed forward (exophthalmos) or sinking into the orbit (enophthalmos).
It is very very important you see an ophthalmologist to find out which of these problems you have. It is very helpful to the Eye MD if you take in some good photos of you taken 10-20 years ago that show the position of your eyes and your eyelids so he/she can see what has changed and that the doctor look at you before the technicians have put any eye drops in your eyes.