Games sales don't go up equally with the install base.
System Sellers usually don't apply when it comes to install base. If it's a system seller/flagship, that means people will play the game at any cost. (that's why they're called system sellers) So most people who wanted to buy the game did. Will the total sales be higher then its GC counterpart? Likely, but not by a lot. Just look at Halo 3, it will probably outsell Halo 2, but only be 1.5 - 2 million. (despite the eventual much larger install base)
Games that benefit from install base are the Tier 2 games, or complimentary games. A game where most people won't buy the system to play it, but will definitely buy it/try it if they ever do get the system.
I think a game like Ratchet and Clank falls into this category. (as an example)
Also, think of who is buying the Wii. It has its hardcore install base they've always had, plus the people who have reverted/gone over to their side. But how much of the rest of the install base is a casual gamer and not interested in Mario Kart/Smash Bros?
However, only time will tell
