1. If you're still on eye drops (with a red top) or were on them recently that can cause it. When they're stopped they pupils usually equalize.
2. If the surgery was months and months ago and one is bigger than the other then the buckling process and sealing retinal holes can cause INADVERTENT damage to the nerves that carry constriction impluses.
JCH MD
It is likely the dilation drops. Did you notice this when you were on the red-top drops?