So, imagine x couples, each of whom has 3 kids in their lifetimes. In 80 years, there are now 3x/2 couples. In 160, there are 9x/4. There are now 11 billion people. Every 150 or so years, having 3 kids roughly doubles the population. (A better model would be a birth/death process with a #kids distribution with a mean 3 and death age with a mean 80, but not necessary to make my point, I think.)

This is exponantial growth.