it boils down to this - a game - any game from a AAA top of the line game to a cheap shovelware game will cost less to make on Wii than the HD consoles.
Take most visually impressive game that the HD console can handle VS the most visually impressive game that the Wii can handle - the Wii's game will require a fraction of the art assets at a fraction of the size because it's the most it can handle. -> Less Man hours = Less $$$ to make the game
Even on the opposite end of the spectrum - I noticed that Boom Blox was mentionned so I'll go with that. The developpers would have to make many more textures much bigger (ie: increased detail) and they're want to put more models and more overall environment for the game to be acceptable on the HD consoles which would multiply the cost of man hours (aka $$$) for the game to be made.
