User blog comment:Xilinoc/Ch. 612 - DIRTY/@comment-3430033-20150126143846/@comment-5398287-20150126153228

Don't take my word as fact here, but I believe it's due to the Weekly Jump adding another series to the roster; since they can only have so many pages per release, they have to cut down on the sizes of some others to accommodate, with the new space going to that new manga. One Piece has managed to stay at 19 pages, but that's likely influenced by its massive popularity.