In that case... then why is the classic old MBP thick?
Because it has an optical drive and 2.5" hard rive, whereas the rMBP has no optical drive and flash memory.

Look at this picture inside the classic MBP. Those two parts are on the left. The optical drive is the rectangle in the top left corner and the hard drive is the one in the bottom left corner. Look how much space they take. The fact that Apple removed them in the rMBP is mainly what let them make it so thin.