Of all the whales that live in the sea, the largest are blue whales. In fact, blue whales are the biggest known animals to have ever lived on our planet, even bigger than dinosaurs. Enrico Pirotta, a researcher at Washington State University, explained that when baby blue whales are born, they are already massive. They weigh between 5 to 7 tons. They stretch about 22 feet long. They grow bigger and stronger, putting on a couple hundred pounds each day.