Why do large whales feed on such small plankton?

ON LAND, VERY SMALL PREY SUCH as mosquitoes, aphids and ants cluster in occasional groups, providing food for other insects and birds such as swifts and green woodpeckers. Larger prey gathers in herds or flocks that occur less frequently. These are preyed upon by medium-sized carnivores, capable of stalking and chasing them.

- Sheena Harvey

Much larger terrestrial mammals, which are often slower and less agile, tend to be vegetarian to profit from more readily available plant food that can't run away from it.