As we have witnessed, 1 million debuts are hard to sustain further on a next studio album. This shows every album that has sold 1 million+ copies debut week, and compares it to the debut sales of the artist's album after (U.S. sales).
ARTPOP it's impact, made me want to see how other artists in that past with huge debut weeks went on in their career. Listed in order from largest fall, to smallest fall:
Artist: Album (First Week Sales) -- Album Successor (First Week Sales) [% Difference]
Lil Wayne: Tha Carter III (1,006,000) -- Rebirth (176,000) [82.51%]
Lady Gaga: Born This Way (1,108,000) -- ARTPOP (259,000) [76.63%]
Limp Bizkit: Chocolate Starfish and the Hot Dog Flavored Water (1,055,000) -- Results May Vary (322,000) [62.48%]
Usher: Confessions (1,096,000) -- Here I Stand (433,000) [60.50%]
Nora Jones: Feels Like Home (1,022,000) - Not Too Late (405,000) [59.62%]
Garth Brooks: Double Live (1,085,000) -- Scarecrow (466,000) [51.6%]
Britney Spears: Oops!...I Did It Again (1,319,000) -- Britney (745,000) [43.52%]
50 Cent: The Massacre (1,141,000) -- Curtis (691,000) [39.44%]
N SYNC: No Strings Attached (2,416,000) -- Celebrity (1,880,000) [22.19%]
Eminem: The Marshall Mathers LP (1,760,000) -- The Eminem Show (1,606,000) [8.75%]
Taylor Swift: Speak Now (1,047,000) -- Red (1,208,000) [15.37%]
Backstreet Boys: Millennium (1,134,000) -- Black & Blue (1,591,000) [40.30%]
Almost all are falls because these huge debuts are where artists tend to peak in their careers. It's all about not trying to freefall after that, and lots of these took long breaks (around 3 years or more) after their huge eras.
Edit:
Here are the ones for the follow-up albums that did one million again! (NSYNC/Taylor not included because they didn't release another album / haven't released one yet).
Backstreet Boys: Black & Blue (1,591,000) -- Never Gone (293,000) [81.59%]
Eminem: The Eminem Show (1,606,000) -- Encore (711,000) [55.73%]
Backstreet Boys released their album 5 years later after fading + Eminem's album was only available for 3 days of a week.