Even if KNY exceeds One Piece sales, we can't forget that it sells less than a few years ago. Here is the Oricon sales of 2013:
2013 Manga Sales Ranking Top 30 (series)
1. One Piece (18,151,599)
2. Attack on Titan (15,933,801)
3. Kuroko's Basketball (8,761,081)
4. Magi (7,151,001)
5. Naruto (5,553,933)
6. Silver Spoon (4,858,699)
7. Assassination Classroom (4,595,820)
8. Hunter x Hunter (4,231,475)
9. FAIRY TAIL (3,790,151)
10. TERRA FORMARS (3,602,630)
11. Kimi ni Todoke (3,428,352)
12. Gintama (2,864,734)
13. Toriko (2,844,248)
14. Kingdom (2,805,945)
15. Bleach (2,752,588)
16. JoJo's Bizarre Adventure (2,588,910)
17. Space Brothers (2,543,140)
18. Saint Young Men (2,487,751)
19. Blue Exorcist (2,329,721)
20. Chihayafuru (2,311,857)
21. AO--HARU--RIDE (2,106,452)
22. Haikyu!! (2,056,241)
23. Ace of Diamond (2,010,045)
24. Detective Conan (1,966,206)
25. SKET DANCE (1,783,098)
26. JoJo's Bizarre Adventure: bunko (pocket-size) version (1,684,749)
27. Tokyo Ghoul (1,666,348)
28. Black Butler (1,562,863)
29. To Love-Ru Darkness (1,558,973)
30. Nisekoi (1,542,417)
2013 Manga Sales Ranking Top 30 (volume)
1. One Piece 69 (3,147,224)
2. One Piece 70 (3,039,487)
3. One Piece 71 (2,890,263)
4. One Piece 72 (2,354,005)
5. Attack on Titan 9 (1,950,807)
6. Attack on Titan 10 (1,915,976)
7. Attack on Titan 11 (1,453,879)
8. Attack on Titan 8 (1,339,845)
9. Attack on Titan 1 (1,299,048)
10. Attack on Titan 2 (1,267,129)
11. Attack on Titan 7 (1,261,251)
12. Attack on Titan 3 (1,250,663)
13. Attack on Titan 6 (1,248,339)
14. Attack on Titan 5 (1,247,333)
15. Attack on Titan 4 (1,241,299)
16. Naruto 63 (1,205,558)
17. Hunter x Hunter 31 (1,193,383)
18. Hunter x Hunter 32 (1,186,245)
19. Naruto 64 (1,088,464)
20. Kimi ni Todoke 18 (1,152,347)
21. Naruto 65 (1,088,464)
22. Kimi ni Todoke 19 (1,069,793)
23. Naruto 66 (1,054,934)
24. Saint Young Men 8 (946,531)
25. Silver Spoon 6 (941,090)
26. Assassination Classroom 2 (910,006)
27. Sakamoro Desuga? 1 (896,968)
28. Kuroko's Basketball 20 (884,432)
29. Kimi ni Todoke 20 (847,264)
30. Blue Exorcist 10 (844,790)