This is an interesting idea mentioned recently by Balaji Srinivasan. I'll expand it here.
- The shapes of current countries are partially (perhaps mostly) due to the constraints of agriculture. Single contiguous areas, in which people grew similar crops and raised similar animals, were zones in which people could move easily and share useful technologies. This often led to similar customs.
- Today, in modern economies, only 1% of the population works in agriculture. Marriage customs, ethnic and cultural boundaries, and political boundaries are much less constrained by the agricultural requirements.
- The hot zone of a polity is its information exchange hub. This is where all other activity is organized.
- This is constrained by _timezone_. Within a single timezone, potential political / economic / cultural coordination is maximized.
- It may now be much easier for a powerful polity to expand _vertically_.
Source:
https://youtu.be/VSVOQl-vFKk