Crusader Kings II

Your population increases steadily over time up to cap determind by your number of empty holdings and capitol upgrades. Conquer enough land and you'll get it naturally. You can also build the upgrades for population growth rate in your capitol to speed up the process. Playing as the Uyghurs in 769 I got this in about 50 years by the time I had conquered everything west to the Caspian Sea and south to the Indian Ocean excepting India (basically modern-day Afghanistan and Pakistan).