The current GP system only makes sense if every non-GP is a little OPM with 100k pops. This leads to absolutely wacky nonsense like the USA sphereing Asia. The problem is that Sphereing as a concept basically means you get dibs on their resources and kind of control their economies- in real life this would be a source of major tension, in game it's just "haha map is blue color now". My opinion- sphereing large countries should work functionally differently than sphereing small ones, if a country has at least 1 million pops then it should trigger an event where the GP sends an ultimatum/"black ships" to demand major concessions (sphereing) from the large minor country in question. This creates a situation where the USA can no longer just go "lol" and sphere all of china, the Russians can't just sphere the Ottomans, the French can't just sphere the Brazilians or whatever and instead provoke a war.
I suppose a secondary criticism would be if a country has a large population it should be able to move people around/recruit pops across province lines in order to concentrate soldiery in one place, so large but expansive countries like Brazil don't end up badly lagging behind and getting beaten up by countries with more concentrated populations. Especially since in real life soldiers aren't deployed to their local town but sent to a base on the other side of the country by central command.
I suppose a secondary criticism would be if a country has a large population it should be able to move people around/recruit pops across province lines in order to concentrate soldiery in one place, so large but expansive countries like Brazil don't end up badly lagging behind and getting beaten up by countries with more concentrated populations. Especially since in real life soldiers aren't deployed to their local town but sent to a base on the other side of the country by central command.