Yeah but even that is not as clear cut, what about nations where over 50% of their population live in one city?
Take South Korea for example. It has a national population of 50 000 000 people (much less than many suspect I think).
Metro Seoul (the largest city) is a massive 25 000 000 people.
So then, using the strategy above, if South Korea were the USA:
Seoul = 160 000 000 people.... kind of a silly way to measure the importance of cities.
It is an interesting method of thought, and does add some insight, but it also goes off the rails at a certain point.
How about Iceland
Within nations themselves same sized metros can have completely different cultural impacts, vibes, etc...
Las Vegas is a perfect example, one of the most famous cities on the planet, but in the USA it is the 30th largest metro area. Even in Canada it would be #4. But, it sure has an amazing amount of fame throughout the world!