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What you are describing is anisocoria (unequal pupil size). The commonest cause for this is any brain problem like infection, tumor, stroke etc. But since you are not having any other symptoms, it does not indicate a brain pathology most probably. The other cause can be increased pressure of the eye (glaucoma), a problem of the optic nerve of the eye.
I would advise you to consult your Ophthalmologist for a detailed evaluation and starting treatment if needed.