There is approximately 15.77B acres of livable land and there are 8.2B people so if each person had just 1/4 acre that would be 13% vs if you gave each person 2000 sqft it would only be 2%. Then you need to factor in how to built transit for low density and how many more stores you need due to the lower density and you can see that it would be much better for the environment if we had higher density