Dave,

6ga? Stock cables are 4ga IIRC

Simple deal, larger wire is more efficient. Most vendors sell wire that is undersized for the applications. If you street drive the car, larger wire is better especially in hot start situations.

I sell 1/0 battery cable and they are stout and do the job with minimal voltage drop.

Weight is not an issue. A 1/0 wire of 15ish feet weighs about 3 pounds more than a 2 gauge wire of same length.

A 15' 2 ga wire weighs 4.2 pounds
Same in 1/0 is 6.9 pounds

You could do 2/0, not really a big step up from 1/0