Note that "arable land" has already been mentioned by the devs as a variable that will be tracked, and used for purposes such as food production. It can be assumed that most of the arable land will be already in use for areas like western Europe (although it would be cool for developments like enclosures and homesteading to be represented too). USA and other American countries will probably have more arable land than their initial population can use.
In my opinion, this discrepancy should be the driving force of emigration and immigration: poor people will migrate to places with lots of free arable land. This phenomenon was the main driver of mass migration from Europe to the US during the 19th century at least. Political rights also played a role, especially where they facilitated easy access to the undeveloped lands, but they are not as important as economic reasons for the lion's share of migrants coming to the US from Europe.