I'm curious about using cable with finer strands. Why does that work better? As a very electrically-challenged person I have no clue. Why do more strands mean less resistance?
The theory I remember from apprenticeship school is the electricity does not travel through the middle of the wire, but on the outside edge of it. So the more strands will provide more area for the electricity in a given wire size.
Something else to think about is the type of wire insulation, there are many different applications. If the wire insulation will not handle the heat of its environment, then you're still not getting the full potential of the battery cranking amps to the starter. A quick Google search describes "welding cable" insulation types as durable and tough. Not all will hold up to petroleum based products. The one heat rating I found was from a Lincoln Welder for 30* Celsius, about 86* F.
From the battery to the solenoid and solenoid to the starter, I ran a #2 type
GXL SGX wire rated for 50 Volts and 125* C, about 250* F. Its suitable for automotive applications and meets some of Ford and Chrysler specifications, according the Wire Barn website.
Knock on wood, no starting issues yet.
Edit, I will add that the insulation is the driving force in how a wire should be used. One aspect is heat. The more amps you push through a wire, the more heat that gets generated. The more heat resistant an insulation, the greater the capacity it can carry for a given size.
Corrected the insulation type for cables