I think this graph does a good job of illustrating it:
http://vgchartz.com/swlaunch.php?reg1=All&game1=Grand+Theft+Auto%3A+San+Andreas+-+PS2®2=All&game2=Grand+Theft+Auto+IV+-+PS3®3=All&game3=Grand+Theft+Auto+IV+-+X360&weeks=100
GTA IV on PS3 and X360 fall about 1 million short of where GTA:SA was on the PS2 at a similar point in its life. The charts look similar, except the two million units GTA:SA sold on Xbox are not shown here.
Yes, GTA IV is not quite living up to its predecessor in sales, but to me the bigger difference is that GTA:SA didn't cost $100 million to make. The series has peaked, and I don't expect investments in future GTAs to be nearly as large.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.
