system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why in some species of animals is the female typically larger than the male, and vice versa in others?
gpt
It depends what the purpose of the male actually is. Females are the staple members of any given sexually reproducing species, because they're the ones that can have the babies. The males have a much more flexible role. In species that have quite small males, the male typically only exists to act as a sperm bank. In species where the male is a *lot* smaller, such as some spiders, there's also other pressures. In spiders, the male tends to be eaten by the female, so a small size is selected for because it makes the male seem less appetizing, which helps it to stay alive long enough to actually fertilise eggs. 

In species where males are quite large, this is often because the males are competing for mates, where large members are more likely to beat small members. Alternatively, the males may play a role in raising the offspring, rather than existing solely to fuck, which is often the case in mammals. In a lot of species where the males are substantially larger than the females, it's because the men have another purpose beyond sex and child raising. For example, in some species of fish, the females spawn inside shells, and the males become much larger because their role is to find the shells - big male can move bigger shell than small male. 

Basically, there are a huge number of different reasons, and you're best off googling the species you're particularly interested in.