Sexual-size dimorphism (SSD) is replete among animals, but while the selective pressures that drive the evolution of SSD have been well studied, the developmental mechanisms upon which these pressures act are poorly understood. Previous research shows that SSD in Drosophila reflects elevated levels of nutritional plasticity in females versus males, such that SSD increases with dietary intake and body size. Additional data indicate that these differences in nutritional plasticity reflect sex-specific differences in how female and male body size responds to levels of dietary proteins and carbohydrates: specifically, while body size responds to variation in protein level in both sexes, only female body size is sensitive to variation in carbohydrate level. Here we explore whether these differences in nutritional sensitivity at the morphological level are reflected by differences in how the insulin-signaling pathway responds to changes in carbohydrates and proteins in females versus males, using a nutritional geometry approach. Our data suggest that there are sex-specific differences in the expression of insulin-like peptides but not, detectably, in the expression of genes regulated by the insulin-signaling pathway. These data support the hypothesis that, in Drosophila, SSD is regulated by the sex-specific differences in the release of insulin-like peptides under different nutritional conditions.