Hi Bob,
Your electrical questions are most interesting on electricity which we cannot see.
If it appears confusing to understand positive (+) ground, negative (-) ground, electricity pushed or pulled, AC, DC, current flow, your jigawatts, hot wires, ground wires, atoms, electrons, etc., maybe think of your Model A electrical system like an organized Male/Female marriage.
1. Unprotected Male: Compare ground to the Male, whether (+) or (-) ground. The ground in not insulated, hence, "unprotected", like your battery ground cable. Because the Male ground cable is connected to the "un-insulated" & "unprotected" exposed metal frame, transmission, engine, etc., all exposed metal is ground & can be considered as an unprotected Male.
2. Protected Hot Female: Compare the insulated & "protected" hot battery cable & hot insulated & "protected" wires, whether (+) or (-), to a protected Hot Female.
3. If the "protected" Hot Female does not touch the "unprotected" Male or vice versa, there are no sparks flying, no excitement, no heat generated; however, if the Hot Female touches the Male, or vice versa, sparks fly & heat is generated. The ammeter can be thought of as a Male/Female excitement meter that monitors the Hot Female touching the Male or vice versa, whereby the ammeter measures the excitement level in amps charging or discharging.
4. The 12 gauge "yellow" wire you mention is an insulated "protected" Hot Female. It connects to other Hot Female wires at the junction box & branches out to things such as light bulbs, coils, switches etc., etc.; however, when the car is not running, & with ignition switch & light switch "OFF", the Hot Female is not touching the Male. For this touching & excitement to occur, the Model A driver has to "make" the Hot Female touch the grounded Male through a switch -- either a light switch, brake switch, ignition switch, or a starter switch.
5. When one hits the starter switch, the wide Hot Female copper spring in the starter switch touches the Male "well" grounded starter. The Male grounded starter is full of metal coils & "well" grounded; hence, there is lots of excitement -- the Hot Female battery cable gets warm, but does not fry because it is large enough to handle this big excitement & connection between the Male & Female.
6. Getting back to the small Hot Female Yellow wire. Look at any Model A light bulb. When the owner creates a "turn-on" at the switch, the Hot Female touches the Male at the light -- but look "inside" the light bulb & you will see an extremely fine wire filament; hence the Hot Female touches the Male very, very lightly through this thin wire filament, so it is more of a slight tickle as opposed the big huge grab that happened at the starter. Because of this Female light touch, "not" nearly as much excitement, so with a very, very light touch, not much heat, & a much smaller wire can handle this Hot Female/Male encounter.
7. In summary, when the starter switch is depressed. the Hot Female touches & grabs the well grounded Male starter with great vigor; hence a large battery cable is required. But when starting a Model A the yellow Hot Female wire is only tickling the ignition switch, coil, & points on the other end; hence, the yellow wire does not get hot enough to fry.
I tried.
Maybe a marriage counselor can add some sentences to explain this Model A electrical mystery.
Hope this helps.