The amount of memory textures can use is not just limited by the resolution. If a game maker really tried hard they could quite easily out do the abilities of even the 64MB memory of graphics cards.
Also these cards may be faster performers, as the GTS did get a performance boost from having extra RAM to play with. Although as many have said, MX is a value part so maybe this is excessive, but now the GF3 is out, maybe some manufacturers are trying to make 64MB cards look attractive.