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Why do eggs usually contain more material than sperm cells?

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Diagram comparing reproductive pathways: arrows show sexual reproduction forming a diploid zygote from oocyte and sperm, haploid parthenogenesis forming a haploid individual, and two types of diploid parthenogenesis-automixis and apomixis-both resulting in diploid individuals. Symbols indicate haploid and diploid stages. — Animal reproductive system The process of sexual reproduction and several forms of parthenogenesis.

In the context of animal reproduction, the difference in size between eggs and sperm is a result of a functional division of labor between the two gametes.1

Male gametes, or spermatozoa, are specialized for mobility and are produced in enormous numbers to maximize the probability of encountering and fertilizing an egg. Because their primary role is to reach the egg and deliver genetic material, they are extremely small and contain little more than a nucleus, mitochondria for energy, and a flagellum for movement.12

Conversely, the female gamete, or ovum, is specialized for the survival and early development of the potential offspring. The egg is typically a stationary, larger cell that provides a store of nutritional reserves, known as yolk. This yolk—which includes proteins, lipids, and fats—sustains the embryo during its initial development before it can fend for itself. The larger the egg and the more condensed its internal reserves, the further the embryo can progress in its development. Consequently, eggs are larger because they must carry the necessary materials to support the new organism's growth.13