As Japanese also conducted their census, here a random post for the 4 most important cities in the world: Tokyo, New York, London and Paris.
I worked with strict and broader metro areas for each one of them and also look into their inner core to look into their densities and urban form as well. Note metro area definitions are two of several definitions I worked for each one of them. They're not comparable as administrative divisions are completely different from country to country. If someone is curious about it, just ask.
Anyway, let's start with the two metro area definitions, the wider and the strict:
---------------------- 2020 ------ 2010 ------ 2000 ------ Growth
Tokyo -------------- 36,938,977 -- 35,618,564 -- 33,418,366 ----- 3.7% --- 6.6% ----- 13,566 km²
New York ----------- 22,692,839 -- 21,358,372 -- 20,675,403 ----- 6.2% --- 3.3% ----- 21,770 km²
London ------------- 18,021,857 -- 16,619,216 -- 15,106,200 ----- 8.4% -- 10.0% ----- 17,586 km²
Paris -------------- 12,496,964 -- 11,871,075 -- 11,214,641 ----- 5.3% --- 5.9% ----- 13,355 km²
* Numbers for London: 2020, 2011 and 2011 and for Paris: 2018, 2007 and 1999
---------------------- 2020 ------ 2010 ------ 2000 ------ Growth
Tokyo -------------- 36,425,363 -- 35,043,522 -- 32,810,253 ----- 3.9% --- 6.8% ----- 10,354 km²
New York ----------- 18,430,255 -- 17,255,469 -- 16,788,583 ----- 6.2% --- 3.3% ----- 10,014 km²
London ------------- 16,006,625 -- 14,744,549 -- 13,395,000 ----- 8.6% -- 10.1% ----- 12,162 km²
Paris -------------- 11,238,820 -- 10,661,748 -- 10,053,456 ----- 5.4% --- 6.1% ------ 5,841 km²
Tokyo is in a completely different level. Almost 40 million in a valley and despite Japan's demographic challenges, it's growing.
And now their core:
---------------------- 2020 ------ 2010 ------ 2000 ------ Growth ------ Density
Tokyo -------------- 12,164,192 -- 11,194,879 -- 10,153,306 ----- 8.7% -- 10.3% ----- 842 km² --- 14,466.8 inh./km²
New York ------------ 9,529,044 --- 8,809,399 --- 8,617,253 ----- 8.2% --- 2.2% ----- 898 km² --- 10,611.4 inh./km²
London -------------- 7,075,358 --- 6,431,279 --- 5,671,500 ---- 10.0% -- 13.4% ----- 966 km² ---- 7,328.2 inh./km²
Paris --------------- 6,824,311 --- 6,542,670 --- 6,164,238 ----- 4.3% --- 6.1% ----- 762 km² ---- 8,955.8 inh./km²
We always talk about New York and Paris when talking about high densities, but Tokyo is also the king here.
Another interesting feature is how the core is growing much faster in all of them but Paris. It's the rebirth of the urban living we've talked about in other threads.
* Tokyo: Tokyo, Kawasaki and 4 small districts; New York: New York City and Hudson County, NJ; London: Greater London minus some outer boroughs; Paris: Paris, Seine-Saint-Denis, Val-de-Marne