Why earth pin diameter is higher than other two pin?

In three pin plug 6 Amp. 220v AC rating. why earth pin diameter is higher than other two pin? what its purpose ?

Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance value decreases)