Robert Trivers developed a theory of parental investment which has been used to make sense of sex difference across the entire animal kingdom. The obligate parental investment is the amount of investment required of a parent to produce offspring which will mature into an organism capable of reproduction.
In humans, a woman is required to undergo 9 months of gestation, and in hunter-gatherer societies we see mothers breastfeeding for another 3 years. Breastfeeding also has the benefit of working as natural birth control because it prevents ovulation. This contrasts against men who need the time it takes to becomes aroused and ejaculate.
With the stakes so much higher for women, they are the more discerning sex, carefully filtering for potential mates. The converse is that men are far less judicious and find themselves in higher levels of competition. This has subsequent order effects on the size and level of aggression of males because the competition would often be physical.