Nikkei Press estimated the market share of publishing companies in Japan in 2015. According to it, Kodansha has the biggest share (9%). The industry-wide total sales were one trillion yen in 2015.
Figure: 2015 Publisher Market Share in Japan
|Publisher||Market share (%)|
Source is Nikkei Sangyo Shimbun newspaper, the article on 2016 July 25th.