Why does my eye look significantly smaller after an injury to the face?

Typically. This would either be due to a droopy eyelid or due to an orbital blowout fracture allowing the eye to sink in. Either way, an oculoplastic surgeon could help you.
Sinking eye. Often a fracture of the bony orbit forming the socket of the causes collapse of the walls into the sinuses and the eyeball will sink backwards into the skull - this is called enophthalmos and is fairly common. Sometimes extreme lid swelling will obscure the eyeball and make it look smaller.