By the way the scientific term for difference in pupil size is anisocoria - and one last thing - sometimes the smaller pupil is the abnormal one.
MJK MD
Could be a trait you were born with, could be due to trauma, adies pupil, horner's syndrome, third nerve palsy just for starters. Some causes are harmless others are very serious. See an ophthalmologist to find out the cause in your situation.
MJK MD