Well, not just Mongolia. When you get in your car and drive across the States you realize how empty most of our country is - we cluster in the big cities and the burbs, but my goodness, there's lots and lots of empty land here. I notice it driving out to MD (east), but it is even more striking when heading west (my brother lives in Montana).
I lived in Wyoming -- lots of empty spaces. However, even with all this empty space, do all the people who are now filling the earth have clean water to drink? Do they have sufficient food to eat? Do they have what is necessary for shalom in their lives?
I remember reading about an experiment with guppies in a fish bowl. In one case, the experimenters started with two guppies -- a male and a female. Over time, they reached x number of guppies in the bowl. In another case, the experimenters started with the same size fish bowl, and filled it with many, many guppies. Over time, they reduced their numbers by killing off the weaker ones to x number of guppies -- about the same number as when they started with two.
If we are allowing people to starve to death, perhaps the earth has reached the capacity population that it can handle.
Lets assume for a moment that this line of reasoning was adopted by the governments of all humans and that at the first sight of famine they crack down on population control. Abram would not have prayed for or accepted the promise of God that he should have children because there was famine in the land and it was severe (Genesis 12:10), so Abram should have told God that there wasn't enough food and good water for the whole world so he shouldn't have any children to inherit the land... And again, God promised Isaac during times of famine that he would have MORE children and the famine is to be endured, but Pr. Stoffregen's method would have had Issac telling God that there wasn't enough food and good water for everyone so he shouldn't have children since the world must be full. And what about Joseph, he must have been such a fool, building up storehouses when what he really should have done was put in place a government enforced population control methodology.
No, you see, none of that is right. The human race has NEVER had a period of time that met the requirements set forth by Pr. Stoffregen, there was never a time since Adam and Eve left Eden that there was plenty for everyone, never a time that everyone was fed and no one starved and everyone had clean and plentiful water. Its a false dichotomy to claim that since we don't have it now for the entire world the world must be full.

Since the Pharaohs or the Kingdom of Ur, from the Stone age, Bronze age, and Iron age, through the Axial age and the industrial revolution, from Chinese to the Incans, the fantasy requirements of everyone having enough food and water has never, ever, occurred. WE, globally, now have a higher percentage of the people with good food and reliable sources of water than any other time in the history of civilization and humanity itself. Now is the best time in the history of the world to have children. The likelihood of being able to clothe and feed them and they can live long and healthy lives is higher now than any other time in history. What irony it is to now claim that since we can't guarantee food and water for every soul born we should produce less born. Funny stuff, self focused absurdities, are.