The population explosion was caused by two things. One was the natural birthrate of the colonists. Partly because of the time-honored farm family tradition that large families meant more people to work (and maybe because there wasn’t much else to do on those long winter nights in the country), the size of many American families was astounding.
But the growth was by no means all from within the colonies. Immigration was the second factor in the population explosion. It continued at a brisk pace, not only from England but also from other Western European countries.