How did whales get so big? Paleontologists say they’ve figured it out.

most baleen whales remained about 15 feet long. But 3 million years in the past, the scientists reported Tuesday in the Proceedings of the Royal Society B, baleen whales underwent a dramatic size shift. The smallest baleen whales vanished. The others grew to double or triple the size.

.. experts previously proposed baleen whales grew large as a way to avoid being eaten. Even the biggest ocean predators from several million years ago — sperm whales such as Livyatan melvillei, or the shark Carcharocles megalodon — would struggle to chomp on anything larger than about 30 feet (smaller than the average humpback whale alive today). But baleen whales didn’t outgrow such carnivores until millions of years after predatory sperm whales appeared in the fossil record.

.. about 3 million years ago, around the same time that baleen whales got huge. First, seasonal windy upwellings began to kick up nutrients along the coast; later, glacial runoff added to the richness of these pockets. The net results were souped-up marine ecosystems where whales could feast.