They've cut waste at the buffet by cheapening the ingredients - you've never seen so much rice as a Carnival buffet, and the sausage on the breakfast buffet is just cut up hot dogs. If you're lucky enough to get actual sausage on your cruise, you'll notice it's just Banquet brown n' serve links. They will cut a chicken thigh into three pieces and that is one portion. They provide smaller plates and don't provide trays (and the kids cups for drink).
If they wanted to, they could make it more cafeteria style and have crew serve the guests from the buffet like the cake buffet is now - but that would probably require more staff, not sure if they want to go down that rabbit hole. But that is probably next.