Physical volume sales in Japan for every manga have been decreasing for years, and digital don't really seem to be recorded well.
Even counting that, One Piece's individual volume sales are heaven and earth above its competitors.
This year it looks like Kimetsu no Yaiba will be the best selling manga in Japan, slightly overtaking One Piece, but it's not so simple as it simply outselling One Piece. KnY is seeing a massive boost due to its anime, and because of that it's backlog is selling well. People are going back and buying the entire manga from the beginning, volume 1 up to 19. That's what's pushed it above One Piece. When it comes to individual volumes, though, One Piece 94, the last volume released, sold 1.7 million. None of KnY's individual volumes have sold over a million.
For One Piece, they had a huge sales boost years ago, where Marineford and Strong World combined. Properly ridiculous sales numbers, with people buying pretty much the entirety of the pre-timeskip volumes.