I felt myself somewhat constrained by my earlier "Co-Prosperity Sphere" ideology, in which Japan justified taking over pretty much everything in the Pacific by claiming it made economic sense to unify these areas. Looking purely at transportation infrastructure & watersheds, those parts of northern Mexico connected to New Mexico or the Rio Grande area don't have much reason to be part of another country. Obviously, other concerns are paramount, but this is an example of ideology trumping common sense. So, too, is allowing a big 'ole country to still exist...
Anyway, the borders are a bit messy, as I'm constrained by the province borders in the game engine.
Rio Grade watershed picture, btw:
http://www.theriograndeaneaglesview.com/great_river_campaign.php
So that's pretty much the Chihuahua and Monterrey border justification.
Edit: I'm not terribly sure about a continent-striding superpower either. Perhaps a few decades into the future. It's certainly not very influential on the world stage. Internally, the divisions should be serious for the moment. To represent that in-game, I did NOT give New England any new cores, so the vast majority of that territory is NOT a national province.