Superman who?
Source
Keep in mind One Piece has over 100 volumes
As far as I’m aware, Superman issues are the length of a typical One Piece chapter, there are over 1000 of them and they cost 10 cents per issue for a significant period of time. The fact that One Piece is very close to the sales when they are 200 page volumes and there is only 100 makes the feat even more impressive.
Do keep in mind that Superman isn't the work of a single artist, but rather countless different artists and storytellers over several decades that have put out all kinds of interpretations of Superman. Some have been really popular, but others have completely dropped the bomb. It's easier for a lot of people to stay invested in One Piece since it has a continuous storyline with one artist's vision being portrayed throughout the entire series. It's surprising to me that Superman even HAS sold as much as it has. Most of its sales must be from back in the day before a lot of other superheroes became popular. I just don't see it selling well today, now that the comics have completely lost sight of what made Superman so popular to begin with.
Even still, this is certainly a very impressive feat by One Piece.