It doesn't make much sense to just compare the two countries by raw population numbers. Denmark has a population density of 130/km2 (or 336.7/sq mi), which is obviously vastly less than NYC, but almost 4 times that of the US. If Denmark was a country with enormous amounts of windmills and small pockets of actual humans, this wouldn't be much of a feat. But for a relatively densely populated country, it seems worth noting.
Though if you actually need land proportionate in size, and with the geography of Denmark - so flat that the highest point is no more than 171m - to service 300 million people, you might be out of luck (I don't know how much of a role this plays).
Though if you actually need land proportionate in size, and with the geography of Denmark - so flat that the highest point is no more than 171m - to service 300 million people, you might be out of luck (I don't know how much of a role this plays).