To add to Feif Un’s point:
In former times most women who nursed their babies (and most women did) did not ovulate until they weaned their babies. Some of the halachos mentioned in the Gemara about nursing mothers are based on this fact. Since most babies nursed for a year at least, babies were born further apart. Of course today many women ovulate while nursing (perhaps because of different lifestyles/ diets or different nursing patterns), and most babies don’t nurse that long, so women can now have babies much closer together than was formerly possible.
Consider that most women in TaNach did not have many children, certainly not as many as many families today (Leah and Peninah are obvious exceptions to this). Most men who had many children had multiple wives and/ or concubines.