Reduced competition, energy loss. Time spent per food patch increases with the distance between patches. Food accumulates linearly with time spend in an infinite patch. Finite patches are foraged to exhaustion. Large whelks take less energy to crack, so the net energy gain is higher.
Small whelks are easier to carry, so the net energy lost is lower.
Large whelks have a more more calories per gram of meat. Small whelks are easier to chew, so less energy is lost.