Induction gives a hood answer. Would your country be better off balkanized with many small territories feuding and redundant administration on every new level? While there are occasional scaling issues there is a synergistic effect to larger unified territories - as opposed to actively occupied ones.
Joining together would be a long term investment for the US. Given how well trade matches as size of neighbors attenuated by distance roughly it would pay off long term. Short term at least a significant minority would probably be discontent about it given what happened just with German reunification and they weren't even apart for a century.
Language and culture both matter when dlooking at a country - the US joining with several countries with a culture of corruption and massive distrust of the government would not likely end up well for the US.
Redundant administration? By that logic, you might as well create a one world government, but it's a piss poor idea.
Consider that redundancy is good in that it creates multiple points of failure. Consider that bureaucracy will scale with the size of the country and not necessarily in the efficient way you think it would. Consider that regional needs and cultures vary dramatically. Even in individual countries, subsidiarian governance is good in that it best matches law and policy with the particularities of each region because local authorities have better grasp of local issues (and where matters are escalated only as necessary). Blanket statements about bigger being better are flatly wrong. And where the US is concerned, the US benefits greatly from a very fortuitous combination of geopolitical factors. Russia, on the other hand, expanded eastward with no such commensurate benefits.
As I said, Italy is already in the EU. Why would the small territories of Italy be "feuding" if they were all part of the EU? And I'm not sure that the administration would be much more redundant than it is now.
On the other hand, you seem to be taking in account only the economic aspects of a unification. Yes, a unification process with poorer countries would be expensive, but maybe that's not the major issue with it. The issue I'm more worried about is that in a democracy the prevalent culture shapes the administration and, of course, the society. Unifying the US with, say, Venezuela, doesn't mean turning 31 million Venezuelans into Americans; it means making the US 10% more like Venezuela. It changes the identity of the country, and you might like the current identity better than the new one.
Joining together would be a long term investment for the US. Given how well trade matches as size of neighbors attenuated by distance roughly it would pay off long term. Short term at least a significant minority would probably be discontent about it given what happened just with German reunification and they weren't even apart for a century.