If you have a load of a motor, and lets say for example it draws 40A at 12V continuous, you then wire another battery in parallel with the first, now each battery will only need to source 20A to the load. So you will still need the heavy gauge wire to the motor from the battery pack, but you now only need 1/2 of the wire diameter between batteries. With more batteries, you can get fancy with the wiring to minimize copper, but I don't need to go there.

With that said, you are likely better-off using the same gauge as to the load for terminal sizes and posts to match up.

What you will find at the end of the day is longer run time, longer charge time, and probably better starting performance as two batteries can better-supply surge current to a starting electric motor.