Your data about BTC exchanges volume ranking is wrong. For you are counting volumes of Net Settlement and Future Trading for bitFlyer. As you can see the correct data on another web site for Japanese,
https://jpbitcoin.com/markets, the volume of bitFlyer is 2nd in Japan, not so big as your data shows. Please fix it by removing the volumes.
We've just fed their volume from the API Bitflyer providing. we'll try to check the issue and inquire it to Bitflyer soon as possible. If any correction or changes, let you know here. Thanks for the report!!!
It seems you are using a wrong key, "volume", for volume. "volume_by_product" is the right one. "volume" includes all volumes of BTC-related boards. Sometimes people used your wrong data for pumping/dumping in Japan. It really sucks.
https://api.bitflyer.jp/v1/gettickervolume: 174115.6117972,
volume_by_product: 25006.19667218