Biologist: Some speculate that the unusually high frequency of small goats found in island populations is a response to evolutionary pressure to increase the number of goats so as to ensure a diverse gene pool  However, only the reproductive success of a trait influences its frequency in a population
So, the only kind of evolutionary pressure that can reduce the average size of the members of a goat population is that resulting from small goats achieving greater reproductive than their larger cousins
Contrary to what some believe, large goats achieve greater reproductive success than small goats even when space is limited.