system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are many female animals larger than their male counterparts but human males are larger than females?
gpt
It all has to do with how selective the females are for mates. Typically in mammals, the reproductive process involves really significant resource expenditures by the females. Plus, the amount of offspring produced in one birth event is relatively low. Cats will have maybe 4-10 kittens in a litter, dogs will have 5-10, and humans will have 1-2. Contrast that with a turtle who will lay 20-100 eggs depending on the species or spiders that lay hundreds. Mammals expend far more resources, time, and energy to produce fewer offspring, so the females are going to be very selective with the mates they choose. When competition among males for females is intense, the larger males will win the fight. As a consequence, animals with reproductive tendencies like mammals (high female selection, social animals, and extensive parenting), tend to have larger males than the females. 

As a contrast to that, animals who produce tons of offspring over a short period of time, expend less resources, and have low levels of parenting will have the females larger than the males. In these types of species, so many offspring are produced that there's little incentive for the males to compete with each other for females. Every male can get the chance to reproduce. Without a selective pressure for them to be large, they will be just large enough to survive to breed.