Not so long ago, most people had large families: five children, on average.
Where once there was one global fertility rate, today there are many, with differences wider than at any point in human history.
Family size, whether small or large, is intertwined with reproductive rights, which are tied to many other rights, such as thoise to health and education, adequate income and the freedom to make choices and non-discrimination.
Where all rights are realized, people tend to thrive. Where they aer not, people are not able to realize their potential, and fertility rates tend to be higher than what most people really want.