I was kind of curious if this was close to true in any countries with higher urban population densities and the first one I checked was Japan since it has a rural depopulation issue and Tokyo is a pretty populous city and… it was right on the money. Japan’s pop is ~124 million and Tokyo’s is ~ 37 million. So roughly 30% of Japan’s population lives in one city/metro area. Not that this means anything for US population distribution, but I suppose it’s not THAT crazy to think the numbers could be in that ballpark if you weren’t really thinking about it too hard.