Natural selection and gamete size.
Natural selection has favored two types of gametes: small and quick (sperm), and large (egg).
For gametes to be successful, they must be able to find another gamete and produce a healthy zygote.
Small (and numerous) gametes (sperm) do the former, while large (and few) gametes (eggs) do the latter.
Medium-sized gametes do neither and are selected against, leading to disruptive selection for small and large gametes.
Thus there is competition among males for the scarce resource - eggs.