> So any theory of "geographical stunting" would need to explain what changed in the geography after 1492.
What changed was technology. Once you have ships going back and forth across the Atlantic, everything changes. Europeans introduced cows, sheep, pigs, chickens. The New World's only domesticable native animal is the llama, which is the main reason why civilizations sprung up in Central and the northern parts of South America, that's where llamas were.
Now, to the Europeans, with vastly superior farming technology, the Caribbean and North America were veritable cornucopias of geopolitical possibility. Islands sheltered plantations. Europe could project real power only very closely to the shore, where the big naval guns could reach. The reason was supply. To maintain a group of soldiers requires not just food but adequate clothing, weapons, ammunition, and horses to carry all of that.
It took over a century of growth for a colony to get to the point to where it could maintain and supply an army for deeper treks inland. By that time the colonies would have been deeply entrenched in native political systems. Because the warships protecting your colony could only hang around for so long, colonies were quite vulnerable to native military action, or even just having them hang around outside the settlement and killing whoever goes outside, say, to chop wood. It made sense for settlers to be nice to the locals, at least at first.
CGPGrey just put out a fantastic video comparing the Old and New Worlds that neatly explains why Europe won and the Western Hemisphere lost. The actual mechanism was disease, which does not require a supply train, but there's a lot more to the story.
https://www.youtube.com/watch?v=JEYh5WACqEk