We know that some species are more sexually dimorphic than others. Species with big differences are polygamous and are called "tournament" species (due to the intense male-male competition) while those with small sex differences are monogamous and are called "pair-bonding" species. (Small) differences might exist across human races as well.
One indication of the degree of dimorphism is the gender difference in life expectancy. According to the NCHS, current life expectancy is 80.6 years for (non-Hispanic) white females and 75.8 years for white males. So among whites, women live 4.8 years or 6.3 percent longer. Expectancies among (non-Hispanic) blacks are 76.5 for women and 69.6 for men. So black women live 6.9 years or 9.9 percent longer. I take this as evidence that blacks are more sexually dimorphic than whites. (Keep in mind that racial differences are reduced since American blacks have roughly 20 percent European ancestry.)