And then, it depends on how many children are born to that increased population. If in every case two people have two children then the population would rise in a reasonably stable way, but when there are more children, then they grow up and produce more children.

For example, if 2 people have 2 children who then have 2 children themselves, the total is 8 (i.e. the couple, their 2 children and 4 grandchildren). If the couple has 9 children then the total is 92 (i.e. the couple, their 9 children and their 81 grandchildren).

Take the second example forward a few generations and there will be a population explosion.