Small fish need small pellets that they can wholly engulf, but they will spend time chasing the biggest pellets. It would be better for the fish if they were given a small pellet they could entirely engulf. They could fill their stomachs instead of scraping off a meal over a lengthy time.
Can small fish eat large pellet? Yes, but that is only by badgering the large pellet around the surface of the pond as it softens in the water, and eating off it like a giant peach.