Probably many people, especially students study the most populous cities of each nation. Then, they study the highest mountain, the longest river, and the largest lake too.
However, not so many people study or try to think where is the second most populous city. Of course, they know that Los Angeles is the second most populous city in the United States. But how about France? Mexico?
May be many Japanese also think that Osaka is the second populous city in Japan but the fact is city of Osaka itself has less than 3 million inhabitants and not so much difference between Nagoya.
The second populous city in Japan is Yokohama, the prefectural capital of Kanagawa prefecture. Kanagawa is also second populous prefecture in Japan, over 9 million inhabitants. Yokohama's population is 3.7 million and it is 1 million more than Osaka. However, Yokohama was not the second place since the history. It surpassed 2 million in 1968 and it became the second populous city in 1978.
Although it is close to Tokyo, roughly 30 km away and not so many foreign tourists will overnight in Yokohama instead of Tokyo. Tourists will rather stay in Tokyo and travel to Yokohama. However, Yokohama has good connection to Tokyo, direct lines to Shinjuku, Shibuya, Ikebukuro and Tokyo station. Then, also closer to Kamakura and Hakone.