This is because men and women have different types of sex hormones that are secreted in their bodies when they become adults.
Female sex hormones are called "female hormones" and male sex hormones are called "male hormones. The increase in female hormones causes a woman's pelvis to develop, her breasts to swell, and her entire body to become rounder and softer.
On the other hand, male hormones make men's bones thicken, beards grow, and the entire body becomes sturdier.
In other words, women's boobs swell up because their bodies secrete female hormones as they grow older.
Therefore, if a man's body is injected with female hormones, his boobs will begin to swell just like those of a woman.