The BigKat solves on of the major problems of the KitKat...that, despite the fact that the KitKat bar had a unique taste, and snapping it apart is fun, it just wasn't filling psychologically. While the BigKat is probably about the same weight in grams as a KitKat, it has a big, square shape that you can fill your mouth up with. In other words, by changing the shape and size, they move the bar on the candy bar continuum from being a confection to being a snack.
The only problem with the BigKat is that you can't snap it in half like a KitKat, and you can't share it with a friend with quite the same amount of fun.