Large lava flows can see individuals always stay on that flow, meaning that they are always selected against if they are light colored. On a small flow, the lighter colored individuals will be selected for when they move, for a time, off the flow to a lighter-colored substrate. This would help keep such individuals/their alleles in the populations on smaller flows.
Work Step by Step
The key here is flow size--use your imagination to guess what differences there might be in selection between small and large flows. Whether individuals spend time off the flow, changing away from selection for black individuals, could be the key to maintaining alleles for lighter color.