it's passed Dragon Ball in sales because it has more volumes/ran longer. Dragon Ball still has greater sales per volume.
sales per volume is true, but DB has the edge in the fact that it came out much earlier.
also One Piece keeps setting record first print runs, and record monthly sales. (but those weren't tracked when DB was coming out)
beyond that if we look at this data taken
from here
we see db has 230 million sales over 42 volumes or 5.11 mil per volume (and is off the market for the most part, though I wouldn't be surprised if it came back this summer)
one piece has 270 over 68 or 3.97 which supports what you said. If we however look at the one piece numbers and look at the current numbers
taken from here
we see one piece is at 345 mil at 77 volumes or 4.48 mil per volume which while overall is still lower than db, is still a massive gain, and if we subtract the number of volumes from sales we get that with the release of 9 new volumes one piece did 75 mil or 8.33 mil per volume since then (obviously some of those sales would be for older volumes but most would be for the newer volumes which wouldn't be surprising given they have had a number of times that the first print runs have been 3.5+miland have sold out right away) which is quite a bit higher than the db sales per volume.
the scary thing about one piece is despite how long it has been going on, it hasn't seemed to reach its peak yet.
I do wish I could find actual sales numbers of the more recent volumes of one piece and of db and what not but I can't seem to find any :/
though of course no matter how successful one piece is, it will never truly surpass DB because it wouldn't exist without DB