My understanding is that pretty much all marine mammals eat a ton of calories relative to their body weight compared to land mammals, and I can't think of an explanation of that besides needing to keep warm in the water.
It makes sense. Building and maintaining that layer of blubber must take a lot of eating for a large animal, and most large whales have their feeding grounds in fairly cold waters.
And of course, building and maintaining a body of 10 or more metric tons requires a lot of food to begin with.
