1994 Hiroshima AG: In 1994, China used drugs so crazily that Japan took most of the recovered gold medals.

‌The 1994 Autumn Hiroshima Asian Games
In swimming, the Chinese male and female athletes used drugs so crazily ㅠㅠ because of this, when they were later disqualified, Japan reclaimed a lot of them.
Still, because China has such a massive population, they had over 100 gold medals, so they barely took any hit (the real hit was for the athletes who used drugs, though, wow)
Even without that, Japan has a population of 125 million, and their swimming and various athletic facilities are impeccable (though Japanese prices are quite expensive. How are we supposed to handle all those admission fees, wow), and they say it is especially important to master the basics like swimming and track and field (running).
Also, since Japan is an island nation, they are fundamentally inseparable from 'water,' so there is also an effort to encourage swimming. Basically...