Retorts involving a country's homogeneity and ethnic make-up as a reason social policies work well in that country usually carry a more sinister unspoken subtext, i.e. that ethnic diversity makes an environment hostile to social policies, and/or that some populations are intrinsically unfit for social policies. I don't want to go too far down that lane, so let's say that guessing the reasoning underlying this argument and examples of populations that argument is typically applied to are left as an exercise for the reader.

It’s not, but this argument comes up for absolutely everything Scandinavian countries do: universal healthcare? America’s too big. Prison reform? America’s too big. Not treating poor people like garbage? America’s too big. Oftentimes there isn’t even any reasoning behind why America’s size would be a problem, which is a good clue that the argument is bunkum.