I think the earmite theory is probably correct. The swelling will eventually go down; but, only when the ear mites or infection is cleared up. When the hematoma goes down your cat will probably be left with a cauliflower ear.
Check pet stores for a liquid medicine you can drop into the ear canal. The liquid will eventually pass the swelling and reach its destination. Massage the base of the ear for a few minutes to make sure the medication makes it all the way to the end and is distributed evenly.
If this doesn't work, you will have to get the cat to a vet somehow because it is then mostly likely an infection.