Does everything run off of one 12ga wire?

[R.O.B.]

My mind is blown right now. From the way I see the wiring schematic, the 4ga battery cable only runs the starter. Everything else is running from a 12ga wire into the fuse box. I currently have no starter hooked up, nor the 4ga cable, but everything else is connected. Sure enough, I've got dash lights, headlights, etc.

Doesn’t it seem like a massive load for just the one 12ga wire? I mean, night, running headlights, brake lights, wipers if it's raining. That seems like a lot.

What am I not understanding?