You can find the paper here.

Demonstrating asymmetric competition in natural systems is difficult, as the effect of large individuals on small ones has to be measured, and vice versa. Numerous experiments have quantified one side of the interaction, typically the effect of large individuals on small ones. Here, we demonstrate, using a long-term study of guppies, that an individual’s performance depends on its relative size, with large individuals being competitively dominant. Accurate prediction of both the mean and variance in body size was possible by using models incorporating asymmetric competition, whereas in models where individuals are competitively equivalent, the predictions were poor.