Why are my eyes different sizes when is smile? (photos)

Thanks for your question and posting photographs.  There is really no explanation of why all asymmetries occur but they are very common and your asymmetry does not need  a surgical correction.  

Your eyes are different because there is a size discrepancy between the two. This is genetic but is being made worse as you age. It is interesting that people who were born without eyes or people whose eyeballs are removed never develop an adequate eye socket. An MRI would probably show the difference between the two. I do not recommend this however as it is likely nothing would be done to correct the discrepancy. A small tarsorrhaphy or canthoplasty on the right would tend to even the eye openings (palpebral fissure).