You need to make the positive battery wire to the front at least two sizes larger than the ground.

If they are the same size and you get a ground in the system they will both heat up like a heating element. If you make the short negative cable in the trunk smaller only it will heat up and reduce the chance of a fire.

I had this happen during a race. The starter came loose and grounded the positive cable. It toasted the negative cable but the positive cable was stone cold. I ran 00 welding cable from front to back and a 4 gauge ground in the trunk. If they had been the same size then the positive wire would have lit up also and been a 20ft burning wire.

If you run a solinoid on the main battery cable to only make the main positive live only while cranking it will eliminate that specific problem once the car is running, but it is still a hazard when cranking.

Don't make the negative smaller make the positive larger.