Hi ,welcome to HCM.Is this the first time you have noticed the size of your pupils in detail?If so it could be just an incidental finding suggestive of
Anisocoria,which is common in 20% of the population.It is physiological and you need not be worried about it.If you have noticed this as a sudden change with other assoicated symptoms like severe pain/light sensitivity/ headache/inability to open your eyelid/
double vision ....then it should be evaluated in detail for neurological cause.
Sometimes this difference can also happen following an
eye injury.If you would like to know the cause,the only way to find out is to have a direct examination, so your doctor can do pupillary examination,check on your
nerve functions and correlate the findings to clear things out.
I have seen many patients walking in anxiously after noticing this size change recently,but the cause will be physiological (which is harmless) and they walk out happily.Since you do not have any severe additional symptoms bothering you,I think it should be Physiological too.
And regarding the pain in your eyelid,could be the beginning of an gland infection/blepharitis/early
stye which can get relief with hot fomentation and oral pain killers.
Hope this answers your question.Take care.