Why is one eye bigger than the other? (photos)

See an oculoplastic specialist for evaluation. At first glance, it looks like your left upper eyelid is more droopy (eyelid ptosis) but there may other factors such as the left eyeball more sunken. The treatment would depend on the exact problem.

Eyelid asymmetries and correctionAsymmetry of the eyelids can be caused by a
variety of problems including drooping of one lid. Asymmetric eyelid folds.
Orbital bone asymmetries can exist.
Drooping or ptosis of one eyelid can cause over lifting of the opposite
side adding to the asymmetries. These
can all be corrected with surgery.

It appears that one of your eyelids is falling lower over your eye than the other.  This is called ptosis.  I would consult with an oculoplastic surgeon to be evaluated.  It is possible a surgical procedure could correct this problem.  

Your left eye has ptosis. This, simply explained, is dropping of the lid. I recommend a consultation with an oculoplastic surgeon for evaluation and treatment. Hope this helps!