Why are calves so large?

Because if the selection for increased size in the beef herd. Many producers see that the only way to compete in the market and make a profit is to have large calves to sell at weaning time. The introduction to Continental breeds like Charolais and Maine Anjou have pulled meat packers in their favor because of their sheer size at slaughter. Other breeds that do not have this huge size have to increase the size of their cattle to compete with these new Continentals, and that means selecting for bigger, heavier calves.