Does your eye hurt? What is your age? Do you have any other medical conditions?
The most common cause of one eye dilating more than the other is contact with chemicals. There could be some irritation from your contact lens or solution. Severe sinusitis can also cause this problem.
There is also an inflammatory condition called Adie's pupil that occurs in young women. We don't necessarily know the cause, but the problem is benign.
Either way, you need to have your eyes examined and you possibly may need a scan of your eye socket and head.
Hope this helps and good luck.