16.5-19.7sqm/person is the range that falls in the median (50%) for Tokyo 23 wards[1]. Only 30% of houses have the minimum recommended of 25sqm/person (so, 70% live UNDER in under 25sqm/person). If the other cities in the graph are correct, that makes Tokyo median size around half of the average of those other cities[2].
So yes, definitely Tokyo housing is tiny. I know it since I live here and talk with people; when I invite someone who is not in tech to my place they all comment on how big my 37 sqm "house" (studio/single room) is, to which I can only agree and laugh/cry inside. I'm happy because I am well for living in Tokyo, but it's still a tiny place compared to my hometown where everyone lives like kings.
[1] https://resources.realestate.co.jp/living/how-much-living-sp...
[2] I believe I'm using median/average correctly here, but happy for corrections! I check "at what sizes it's 50% of the # of households" and then took that measure. Sorry for mixing medians and averages, but I cannot calculate averages with the numbers I found.