Why do darter species that produce larger eggs have lower gene flow among their subpopulations within a stream

There is a distinction between large and small darter species.

Explanation:

The large size darters can produce small eggs and the small size darter can produce some large eggs so the large sized larvae get hatched and emerged out from the large sized eggs and they get dispersed to the short distances in the stream. Hence, the population of large and small sized darters get genetically isolated from each others as they are incompatible for mating and producing offsprings.