The smaller the number, the thicker the cable
eg. 4 gauge is twice as thick as 8 gauge
The thicker the wire, hence the higher the surface area.
From the equation Resistivity = (roh X L)/ surface area
Thus, the surface area is inversely proportional to the resistivy, ie. bigger surface, less resistive.
There are some gauge chart on the internet for car audio, indicating which size of wire is most suitable according to the current of each audio system. You might want to try and surf google. Good luck!
anyway here is one:
http://www.the12volt.com/info/recwirsz.asp