When batteries are joined from positive to positive, and negative to negative, this is called parallel. Think of it as a two fences. One for the negatives, one for the positives. They are parallel to each other and never cross over each other.
Electricity going through wires generates heat and that heat dissipates out of the wire losing that energy. The longer the distance of the wire, the more energy that is lost. To preserve that energy transfer, the farther you go, the thicker wire you need.