Why is one of my eyes smaller than the other, especially when I smile? Self conscious about this. Any way to fix it? (Photo)

Your photos are not head on or straight (head is tilted or turned) from what I can gather your left eye muscle seems to be working more actively.  If I'm correct, Botox is often an option that you can explore for treating the operative muscle.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search. 

It's possible the eyelid closing muscle is working more on left side, making that side appear smaller when you smile.  From your photo, it's also possible that you have mild eyelid ptosis on the left which is frequently associated with contact lens use when it occurs in young people.  While it is common to have mild or even moderate facial asymmetry if it bothers you then it may be worth pursuing a consultation with an oculoplastic surgeon who can give you a more specific diagnosis and offer expert advice. 

You likely have left upper eyelid ptosis. See an oculoplastic specialist for evaluation and possible eyelid ptosis surgery. See following website link. Skype consult available.