American families in the 1700s typically were large. Both husband and wife worked the farm and large numbers of children were needed to maintain agrarian productivity. At the time, roughly 36% of families consisted of 7 or more persons.
The Industrial Revolution saw workers begin to migrate to urban areas, separating work from home for the first time. Children became economic burdens rather than contributors. And by 1900, just 20% of families were that large.
In the 1930s, the Great Depression triggered a further drop in the birthrate so that barely 11% of families consisted of 7 or more persons.