if the correct answer to the problem is C, i.e. we can take all sheep with maxRuns runs or less when the boat has capacity C, then we also can make it with all capacities greater than or equal to C + MAX_WEIGHT,
where MAX_WEIGHT is the weight of the heaviest sheep
If such a boat were less full, then there would be space in the boat more than or equal to MAX_WEIGHT (and can put a sheep there). Since all the boats are at least as full, if we require more boats then the total mass of sheep must have increased.