There is a relationship for sure, but there is a lot more to the OCI than just the sump size. European cars are all bad examples because they require oils that meet their extended drain interval specifications.
Plus, I think that having a large oil sump is actually detrimental in the cold winter months or for short trips around town, as more oil will take longer to heat and the larger sump will accumulate more moisture.
Large sumps, on the other hand, are very good for vehicles that operate long hours or at high RPM. More oil means more stable temperatures and of course more capacity to deal with contaminants, hence longer OCI.
In the end it is a balancing act. Both small and large sumps have their advantages and disadvantages depending on the vehicle and the duty cycle.