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).
Essay Pro