Population was important for development in our world. Specifically, the population of poor people. In Western Europe before the great plague population was relatively stable. After the plague, though , there was a severe shortage of peasants. Those that survivied were a valuable commodity and they could begin to wring concessions out of their masters such as the right to move. Some were able to move toward middle class and this increased the shortage further. Labor-saving devices were adopted more rapidly in order to feed the population. A similar situation appeared in America - too much land and too few people. Contrast this to China, which began to stagnate as soon as the emperor made the decision to stop looking outside his borders. The population was stable and there was no reason to innovate. After all, why use a fancy ditch-digging machine that can replace 20 men if you still have to feed those 20 men anyhow?
Or so I've been told.
And never overlook the value of a frontier (a place where the unhappy can go to be away from society).