Why is my right eye smaller than my left eye? Can this be caused by a serious problem?

You probably have a ptosis of the eyelid.  During a surgery on the eye, a speculum is placed to help keep the eyelids open.  In people who have a weak muscle, this may stretch the muscle a bit more so the eyelid droops.  Most often however it is just something that happens.  You need to see an oculoplastic surgeon who can determine why your eye appears  smaller.   

Eyes can look different sizes for many reasons. Sometimes it can be caused by prior surgery. However I would have this looked at by an Oculoplastic surgeon. This is a doctor trained as both an ophthalmologist and a plastic surgeon. He or she will be able to tell you if this is a serious problem or not. 

Since almost everybody’s face has a larger and a smaller side,
it is not unusual to have one eye bigger than the other. Your
pictures before and after eye surgery might give us some kind of idea
whether or not there’s a connection. An oculofacial plastic surgeon would
be able to take measurement and perform certain tests to diagnose your
condition.

There can be many causes of eye asymmetry.  Most of us have underlying facial (bony) asymmetry, which is normal.  There are, however, certain orbital (eye socket) or sinus conditions that can cause the eye to be pulled inward and/or downward.  If you have concerns or any history of sinus related issues then you could check with an oculoplastic surgeon.  They have measuring devices to measure if the eyes are sitting in the eye socket similarly or if one is bulging or being pulled inward (enophthalmos).  When they did your myopia corrective surgery, sometimes the eyelid speculum can cause the a weakening of the eyelid muscle/tendon that opens the eye.  Best regards.  

Most often an eye looks smaller because your upper eyelid is drooping.  This can happen by coincidence but is a little more common following any type of eye surgery (even if everything is done properly).I suggest you see your eye doctor to be sure that is the problem and everything else is ok.  Then you can decide if it makes sense to see an eyelid surgeon to discuss the possibility of correcting the problem at some point.