These are the reason why Earth pin is thicker and longer than the live & neutral pins.

1) To prevent us from Electric Shocks due to leakage current : When we insert a plug into socket, the earth connections are the first to get established. This makes sure that any leakage current must gets discharged safely to ground. When the connections are disconnected, earth pin is the last to get disconnected. Its for safety of the user.

2) Earth Pin is made thicker so that even by mistake it didn't get inserted in live or neutral wire.