One eye appears to be a lot bigger than the other eye. What do you think the problem is? (Photo)

This is a common question.  Every single person is asymmetric throughout their body and this includes the eyes. Asymmetries and eyelid wrinkles can be corrected with blepharoplasty or cosmetic eyelid surgery. It is one of the first cosmetic surgeries people usually get. While the average age might be 30 to 50's, some patients get this done in their 20s. It really depends on your unique anatomy as well as what exactly is bothering you. That said, even after surgery I always warn patients that there will still be asymmetry and things will not look 100% perfectly even because some asymmetries cannot be corrected unless you  reposition bone, remove muscle etc. and some things are just not worth doing.
Botox injections as well as laser treatments can also be a tremendous benefit if you are not interested in surgery. Of course, there are also many combinations.I would recommend that you see a board-certified plastic surgeon in your area that is a member of both ASPS & ASAPS and performs a lot of eyelid surgery but also offers all the nonsurgical options such as injectables and laser treatments. Go over your concerns with them and make sure above all, you walk out of there with several options and an idea of the pros and cons to each of them as well as realistic expectations. I hope my answer helps and if it does, please take a moment to up vote this answer on the “respond” button on the top right.  Have a great day.

You appear to have ptosis of one of your upper eyelids.  This occurs when a muscle in the upper eyelid weakens and causes the eyelid to fall lower across the eyeball than it should.  Ptosis can be repaired with surgery.  I would see an oculoplastic surgeon or a surgeon experienced in ptosis surgery.  

Looking at your photo, the left eye does appear to be slightly more proptotic (i.e. it seems to stick out more from the eye socket) compared to the right. Have you ever had issues with the sinuses, sinus surgery, nasal obstruction, and/or facial trauma resulting in facial fractures? I would recommend consultation with an Ear Nose and Throat specialist to further work-up why you might be developing this difference in the eye. Hope this helps!