Murder

"beefy enough to handle the current that deals with the starter motor". It isn't and it doesn't. As you noted, the negative battery terminal is connected to the head or block of the engine by an appropriately sized cable. By virtue of being mechanicaly connected to the engine the starter is connected to a massive ground, thereby completing the circuit. The small cable you point out is not involved in that circuit.

As this link no longer works, could someone provide the email address ?