Why only small fish eat plankton?

This assumption is not accurate. Plankton are a diverse group of organisms of various sizes that drift through the water. They form an essential food source for many aquatic animals, including various sized fish, crustaceans, seabirds, and marine mammals. It is not limited to small fish. Different species of fish, depending on their feeding habits and size, may consume different types and sizes of plankton. Plankton encompasses zooplankton, which includes small crustaceans, larval fish, and other invertebrate animals, as well as phytoplankton, which are microscopic plant organisms. Both zooplankton and phytoplankton serve as a fundamental food source for a wide variety of aquatic organisms, spanning various sizes and trophic levels within marine and freshwater ecosystems.