Yup. The gap between #1 and #2 were pretty damn huge before 2013. Attack on Titan was able to close the gap last year with just around 2-3 million difference.
OP volumes sales definitely dropping big since 2012.
Volume sales:
2013: 18,151,599
2012: 23,464,866
2011: 37,996,373
2010: 32,343,809
2009: 14,721,241
2008: 5,956,540
I don't pay much attention to that stuff anymore but during its peak years old volumes were selling a lot, which meant there was still new people starting the series. If sales of volumes has decreased, I think it has more to do with the fact that there isn't as many new people coming anymore.
I've just checked 2013 vs 2011 and this is how new OP volumes sold:
1 3,147,224 One Piece 69 (2013/03)
2 3,039,487 One Piece 70 (2013/06)
3 2,890,263 One Piece 71 (2013/08)
4 2,354,005 One Piece 72 (2013/11)
01 One Piece Volume 61: 3,382,588 copies
02 One Piece Volume 62: 3,207,568 copies
03 One Piece Volume 63: 3,073,175 copies
04 One Piece Volume 64: 2,652,700 copies
The difference is not that big between those 2 years. So the difference is how well old stuff was selling.
For what I've seen Attack of Titans is The Shit right now. Just wow.