Car battery cable size is determined by a variety of factors. First and foremost is how long the cable is. The second most important factor is how much current the starter will be required to draw to turn the vehicle’s engine over for starting. If the vehicle’s battery is on the opposite side from the starter, the positive battery cable will need to be much longer than if they are on the same side. This extra length will create more resistance, so a larger gauge (smaller numbered size) wire is used to have the least amount of resistance possible. Higher starter draws will also require a larger gauge wire.
For more info, see Car Battery Cable Order: Which to Connect First