I have different eye shapes. One is smaller than the other. What could have caused this and how can I "fix" it? (Photo)

The ptosis is worse on the left side.  This can be corrected with the appropriate surgery to improve you appearance and symmetry.  There are a number of wonderful oculoplastic surgeons close to where you live.  The American Society for Ophthalmic Plastic and Reconstructive Surgery maintains a geographic directory that can help you find a highly qualified surgery to assess your concerns.

Everyone has facial asymmetry. No ones face from one side to the other is the same. In your case, it is visible...but not drastic...not drastic enough that I would expect anyone in real life to question. From your one picture...I cant see anything obvious to offer you surgically. For fun, grab a celebrity magazine and look at closeups of people faces and then compare there eyes and the halves of their faces...I bet you see more asymmetty than you have ever noticed before. 

While facial asymmetries are common. You have slight ptosis of your eyelids bilaterally have your eyes evaluated by a board certified oculoplastic surgeon.