http://vgchartz.com/swlaunch.php?reg1=All&game1=Grand+Theft+Auto%3A+San+Andreas+-+PS2®2=America&game2=Super+Mario+Galaxy+-+Wii®3=All&game3=&weeks=20
That is the more correct link for comparing SMG to GTA:SA.
Or, since GTA:SA didn't sell a lot in Japan, you could do this: http://vgchartz.com/swlaunch.php?reg1=All&game1=Grand+Theft+Auto%3A+San+Andreas+-+PS2®2=America&game2=Super+Mario+Galaxy+-+Wii®3=Japan&game3=Super+Mario+Galaxy+-+Wii&weeks=20
and add up SMG Jap + America, since that would be how much they both sold without Europe.
That is, at SMG's time in USA atm, 3.9M SMG vs 5.6M SA. The favour is even bigger for GTA:SA because 43 % of its total sales were in Europe, while SMG only has 30 %. Thereby you could add it up, and see that fictionally, GTA:SA was at 8.008 while SMG is at 5.07.
We can thereby sum up that GTA:SA sold roughly 60 % more than SMG at the time.
Further on, we can add that if we take away the first 5 weeks, SMG loses 2.1M and ends at 1.8M from week 8-12 while GTA:SA loses 3.8M, meaning that from week 5-12 it sold 1.9M. Both games had christmas in this period.
What also should be mentioned, is that GTA:SA launched in late 2004, meaning it only had 2 years of sales, while SMG will have minimum 4. As Galaxy is likely to be a long-legged game, it will sell the last 2 years too.
If we then assume that Galaxy manages a 5K/week for worldwide in those years, it will catch 500K there.
It should also be said, that this comparison is slightly unfair, because Europe is a legged game, so while GTA:SA get's the legs added in here as overall, SMG doesn't. That means we are a slight bit in favour of GTA:SA in this comparison. We can thereby say that GTA:SA is maybe only 2.5M in front of SMG.
Feel free to comment what I said.
EDIT: This was meant to be in your other thread :P
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS