when a fault occurs a large amount of current is drawn from
earth wire hence  more thickness is required to avoid
insulation breakdown

As Leakage current need to flow through the Earth Wire
the resisitance should be as low compared to that of the 
phase and neutral wires.Besides,The resistance of the 
conductor is inversely proportional to the cross sectional 
area of the conductor .Hence bigger cross sectonal area 
reduces the resistance and provides a easy path for the 
current flow.So the Earth Pin is always bigger than the 
other two pins in a 3pin plug.

as we know that R=p*l/a 

resistance of the 
conductor is inversely proportional to the cross sectional 
area of the conductor .Hence bigger cross sectonal area 
reduces the resistance and provides a easy path for the 
current flow.So the Earth Pin is always bigger than the 
other two pins in a 3pin plug.