One eye bigger than the other?

Hello!
My name is George. I am watching this forum for some time: great place for informations and advices.
I came writing now because i think my 6 months INR has a problem.
I just noticed couple of days ago that he has one eye bigger than the other... As i said this dates from couple of days ago - 3 days max. It is a matter of concern or it is something normal at his age?
He is my first INR but not my first bird. Haven't seen something like this in other birds i had.
I'm asking here first because i don't want to put him on the stress of a vet "inspection" if it's nothing serious.

Thanks you and sorry for my bad english!