Females tend to have smaller lungs primarily because they experience a lower rate of alveolar multiplication compared to males during early childhood.
While the exact reason for this difference in alveolar development remains unknown, the difference in lung volume appears to be established in the first few years of life. Alveoli are the tiny air sacs in the lungs where oxygen and carbon dioxide exchange occurs, and a lower number of these sacs directly translates to reduced lung capacity.
Therefore, the difference in lung size isn't primarily due to lifestyle factors or conscious effort, but rather a developmental difference occurring early in life where girls experience a smaller increase in the number of alveoli compared to boys. The cause of this difference in alveolar multiplication rates is currently undetermined.
