Excellent photo of your problem!
Your eyes are actually close to the same size, but you DO have a significantly drooping upper left lid (and maybe some droop of the lower one, too). The *visible portion* of the eye is thus smaller than usual.
This eyelid droop is called 'ptosis,' and you can google the term for a lot more information.
There are quite a few causes, from simple heredity to several types of diseases. If there are associated diseases, they should be treated.
In any case, as you will read, ptosis can be pretty well corrected by surgery to give the face a much more symmetrical appearance.
Having smaller eye than the other is probably due to the weakness of the muscle of the eye. This condition is called ptosis or drooping eyelid. It is not actually a disease, but a symptom of another disease. The treatment depends on its causes, that is why it is very important when you visit an eye doctor, you need to tell all your history regarding your condition.