It's been commonly noted that the iPhone 4S looks the same, but that its internals have been upgraded. Despite that, the iPhone 4S weighs more.
What causes the added weight?
Just because internal hardware has been upgraded, this does not mean that the phone will not weigh more.
After all, upgraded hardware is different hardware.
Still, if that is not sufficient to you take a look at the teardown of the two phones. You will see that the internal design differs, which can cause weight difference.