The evolution of genital size in mosquitofish

Why do genitalia show such a precise allometric relationship with body size in these fish?

We are carrying out artificial selection on genital size to explore the fitness consequences of changing the allometry of the size-genital relationship. Is the observed tight allometry due to 'developmental constraints' or does it reflect selection on allometry? If the latter, what type of selection? Natural selection for swimming performance and/or sexual selection for copulatory success and insemination efficiency?