Each generation doesn't have to be bigger than the last. Two people create two children, those two children create two children, the grandparents die, the grand

Each generation doesn't have to be bigger than the last. Two people create two children, those two children create two children, the grandparents die, the grandchildren create two children, etc. That stabilizes the population (technically population goes down due to accidental deaths, diseases, etc. but you get the idea).