Why does one eye look smaller? (Photo)

Looking at your photo, the right eyes appeared to be slightly smaller. You have a mild form of ptosis. The right eyelid set slightly lower than the left. They’re about a millimeter difference between the two eyes, The right upper eyelids sits a millimeter lower than the left eyelid. Your condition is very common and most people would choose to do nothing about it because It’s a small difference. This can be addressed with ptosis surgery to lift the right eye higher.

The pictures are a little distorted so hard to see clearly, but from your description one eye might have subclinical ptosis. This is correctable with blepharoplasty that address and tightens the levator muscle attachment to the tarsal plate. The recovery is maybe 1 week longer than skin only blepharoplasty.