It requires a very different 20th Century;
With a POD after 1901, how could America grow much larger? I'm talking like large scale annexation.
Maybe the US annex Germany and Japan after ww2?
Even as early as 1899, there were decision-makers in the US who thought the Phillippines, Puerto Rico, etc were liabilities (as the PI turned out to be)...
There was also a recognition that governance would lead to statehood, if the concept of the US as a federal democracy was to remain the foundation of government; add in the fact that economic dominance was (generally) a lot cheaper than political/military dominance, and there was a lot of resistance to further territorial aquisitions, and a movement more towards partnerships with the Latin American republics.
So, having said that, I'd think you'd need a different 20th Century, and the most obvious difference would be one in which European great power politics left a potential vacumn in the Western Hemisphere - the obvious route for that is a Anglo-Continental Power confrontation that becomes stalemated and lapses into a cold-to-warm conflict that lasts for decades...
So, depending upon the turn of events and domestic politics, I could see the Western Hemisphere territories of the occupied European powers be annexed by the US and/or Canada and the UK; likely candidates are Greenland, Iceland, possible the French, Dutch, and Danish West Indies, St. Pierre and Miquelon, etc. None are that large in terms of population, but territorial or commonwealth status is possible, at least for some.
After that, it would be whether Canada (and Newfoundland, if it has not already joined the confederation) was so threatened that they came in; that basically requires the UK to go do fighting, which is pretty ASB-ish, but given an early enough point of departure and use of of NBC weapons, is at least in the realm of the possible.
Latin America seems pretty unlikely for political reasons, both international and domestic, after 1900; white supremacy being the foundational one. One minor possibility would be for a diplomatic settlement that splits the Colombian department of Panama, with the CZ becoming sovereign US territory; another possibility is that Panama east of the CZ remains Colombian and west of the CZ is annexed into Costa Rica (or a Central American federation).
I'd say anything in the eastern hemisphere is really out there.
Best,