Why an egg is so much larger than a sperm cell?

In mammal eggs, the food is only sufficient for the egg to nestle in the uterus wall until a placenta is formed; in egg-eggs, such as those found in birds, the food is sufficient for the chick to grow into an independent being.

The egg is larger than the sperm cell because it contains abundant cytoplasm and organelles necessary to support early embryonic development, including energy production, protein synthesis, and cellular division. Additionally, the egg carries nutrients and maternal factors essential for the initial stages of development until the embryo can establish its own support systems. In contrast, the sperm cell's primary function is to deliver the paternal DNA to the egg for fertilization, requiring only a minimal amount of cytoplasm for motility and a streamlined structure for efficient movement.