Is the fact that my optic cups are not the same size reason for concern?

Depends: If an isolated issue discovered by an ophthalmologist, then it might be how yours have developed. Disparate size can be an early indication of susceptiblity to glaucoma which is quite treatable. But you have already seen an eye doctor (be sure it was an ophthalmologist - an md) who can do the other testing needed to rule out this condition.

Not necessarily: Could be anatomic but should be checked out. It's more concerning if the cups used to be the same size and now have changed thus "asymmetric enlarged cupping".

Not necessarily: Difference in size between the optic cups can be significant but usually not in isolation. Other findings suggestive of optic nerve damage have to be present in order for concern. Typically, if there is a significant difference in optic cup size with another risk factor for optic nerve disease (family history of glaucoma, high eye pressure, age etc.) testing will be done to rule out damage.