To a small extent a difference in pupil size is normal, and may vary from left to right over time. However, if there is a larger discrepancy in pupil size it may indicate or warn of a neurological issue.
The determination of the causative of discrepancy in the size of pupil will need following:
1) A physical examination by a neurologist and eye specialist.
2) Fundus examination
3) CT or MRI of the brain and eye balls
Anisocoria (uneven pupils) is condition where one pupil is larger than the other. the causes can be trivial to some which will need medical intervention. Following are the causes;
1) Physiologic anisocoria; there is no vision loss or double vision, so nothing is to be done. The size discrepancy is usually 1 mm.
2) Horner syndrome; due to sympathetic dysfunction of the nervous system. There is associated eye lid fall. Will need further thorough investigations.
3) Third cranial nerve paralysis; blood vessel aneurysm or tumor may cause it.
4) Damage to Iris of eye; should be conformed by eye specialist.
6) Tonic pupil
So an examination is essential for pin pointing the cause.
Hope this helps.
Your follow up questions are welcome.