No secret. When the rock is made it is a much larger diameter than when on sale in shops.
The outer (pink ) layer is a rectangle laid flat and the core with letters are fashioned from strips laid on it. The whole lot is then rolled into a big cylinder and then rolled (like you used to do with a plasticine sausage) until the required diameter is reached. The rock (and letters) get stretched in this process so as the length increases the diameter (and letters) get smaller.