It's probably because Paris, officially, is the very small hypercenter of the Parisian metropolitan area. Whereas Tokyo is the actual total metropolis. Just look up the population of both. Paris is 2 millions whereas Tokyo is 14 millions. If you were to consider the whole Paris metropolitan area then the population is 11 millions.

Yes but the average size of a flat in Paris would be shocking to an American.

500 square feet for a couple is standard (that would be seen as a small appartment in the USA) and living in 300 square feet or less is fairly common for someone single.

To be fair, the average size of a flat in Paris is shocking to Frenchmen living outside the city.

Same thing with New York I guess. I had a cousin move from Paris to Manhattan, and it didn't really feel like an upgrade.

tokyo has 20x as much land as tokyo. If you just counted tokyos densest 40 sq miles it would be denser than paris