The wire is larger in a PV "SOLAR" system because of line voltage drop, the further the distance the more drop
A 100 watt PV panel is probably a 17 volt panel , watts divided by volts is amps , 100 divided by 17 = 5.9 amps , that can surge a small % higher than that in extreme condition
In
thischart , third one down , you can see why you have to use larger wire
With one 100 watt panel and a 10 amp charge controller Id use a 10 amp fuse within one foot of the battery , a larger one would work but wouldnt trip until 30 amps was exceeded