Simply put , if all the current required cannot pass through the circuit the cables will overheat and that heat ADDs resistance and reduces the current available to cover the load, it's all interrelated.

Generally though the Ground cable doesn't need to be as large as the Positive cable so long as the length is short and it has solid continuity/connection to the frame.

1/0 AWG welding lead is great cable for line/load connections, it is fine stranded and when used with Crimp connectors makes for a solid air tight connection for all the electrons to flow.

Especially with low voltage systems any resistance at all decreases the energy (current) available dramatically. It's not as much the size of the ground but more the quality of the ground.