Nintendo (like most Japaneese developers) has their own internal tools and technology team which builds their 'engines' ...
Beyond that, Super Mario Galaxy has very different technical limitations that can be exploited than Super Smash Bros. which means that (although they're both very good looking highly detailed games) the impressive feature of one game may not be easily possible in the other game.
The only other thing to consider is that how hard a game really pushes the underlying hardware is heavily determined by the artistic directors desire to push the underlying hardware; if you think that the overall quality of Mario Kart Double Dash was good enough, and don't really see much of a need to produce noticeably better graphics you probably won't.
