A city that has a height restriction of half of the other city can have exactly the same floor space to live in per person while ostensibly being half as population dense. And height is just one of many factors. Large bodies of water, parks, large open squares etc... they all can change density figures a lot without any change to the actual space per person in an actual house.

Paris certainly has tiny homes, I've been in more than a few, and they've felt smaller than Amsterdam (where I live) generally. But it's nowhere near the discrepancy that the density figures suggest (e.g. 21k vs 4k, but floor space per person is nowhere near 5x as big in Amsterdam, probably more like 25% or something). There are all kinds of reasons for this, one is that Amsterdam hasn't got nearly as many tall buildings as Paris does, and that Amsterdam's density is understated due to quite significant bodies of water (two rivers splitting the city vertically and horizontally), parks (Amsterdamse Bos is 3x Central Park), lakes etc etc, where nobody lives but that is part of the density calculation, making the actual places people do live denser than they appear in these numbers.