Well you also have to consider that Nintendo actually also supports their handheld (unlike Sony) side with lots of a internal development. So the actual break down is a little different if you look at it that way ...
NSMB (2006) Super Mario Galaxy (2007) NSMB Wii (2009) Super Mario Galaxy 2 (2010)
Zelda: TP (2006) Zelda: Phantom Hourglass (2007) Zelda: Spirit Tracks (2009) Zelda Wii (2010/11)
Animal Crossing (2002) Animal Crossing: Wild World (2006) Animal Crossing: City Folk (2008)
Mario Kart Double Dash (2003) Mario Kart DS (2005) Mario Kart Wii (2008)
Though I would agree the incremental improvements in Animal Crossing are lame.
Nintendo does need more studios, but they are also in the process of building a massive new R&D/development expansion in Japan (they purchase a large chunk of land last year for it). You probably will not see the benefits of this expansion until the next cycle of Nintendo platforms though (DS2, Wii 2).