Here's a more fair comparison...
Halo 3
2007 6,835,323 N/A 6,835,323
2008 2,046,996 -70.1% 8,882,319
Halo Reach
2010 7,059,031 N/A 7,059,031
2011 1,561,466 -77.9% 8,620,497
Halo 4
2012 7,197,135 N/A 7,197,135
2013 1,490,851 -79.3% 8,687,986 and growing.
binary solo said "Each successive Halo title on Xbox 360 has sold less than its predecessor, yet with a larger install base"
Userbase has no direct correlation with sales. Mario Galaxy 2 sold less than the original, Killzone 3 sold less than 2, Forza 4 sold less than 3, Black Ops 2 sold less than MW3, etc.
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