1. Japan's urbanization stopped long ago,
2. and it's not taking in immigrants fast enough,
3. so the urban areas have stopped growing.
2 is pretty infamous unless something big happened recently (a lot of big things in JP happened recently, so I could have legitmately missed something).
1 is 50/50. Urbanization is growing because the small town life is shrinking.it's wrong at face value, but there is a cost to this in the overall economy, since the country overall isn't growing.
I thought the Tokyo megalopolis was still growing due to internal migration, at the cost of depleting literally everywhere else in the country.