On land, vertebrates that are the biggest are usually prey items (elephants and giraffes). Usually predators that are larger are smaller than their prey (like lions are smaller than most of their prey items). Evolution happens by there being an advantage to something. Given baleen whales and toothed whales are usually eating prey items smaller than themselves and are predators, what advantage did getting larger have? Like blue whales or humpback whales are feeding on tiny krill and shrimp, but they’re massive.