the old Xbox controllers weretoo big, thats why they did the special edition ones like you see below:
they were origonally made for the crystal but they did duel shock ones that basically looked the same but with different colours.
now the new Xbox 360 Controller is basically in between the sizes because people complained on the origonal Xbox that they were too big and people who were adult and found the big pads too big thought that the smaller pads were too small, so microsoft came up with the brilliant idea of making the pad a more suitable size like below:
plus you can personalize your console more with the new 360 as you can now get faceplates that you can create yourself and create different designs for your controller, like stickers but of course things like that don't come free unfortunately.