system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why mammal females often are smaller than males?

Females have to carry fetus and protect themselves and babies, why are they often smaller than males?
gpt
Mammals typically live in groups, and males typically compete for mates.  Both this competition and the need to defend territory and protect the group leads to natural/sexual selection favoring typically larger males.  A larger male can defend against threats better, can fight other males for mating rights better, and females who want to choose the best mates will favor them.

Females don't have as much luxury in being able to grow larger/stronger/etc., because their bodies' effort goes more towards rearing children.  A male only has to survive long enough to bang, basically, so his body can be large, energy-hungry, and shorter-lived.  A female has to be able to devote a significant chink of her effort and nutrition and time into raising young.

(Note this doesn't hold true always, or as a blanket rule.  It's how it works out for many species, but the ins and outs of sexual dimorphism go way deeper than this.)