Why is the earthing pin in 3 pin electrical plugs bigger than the other two pins?

The earthing pin in any 3-pin electrical plug is bigger than the other two pins due to the following reasons.

The earth wire starts from the metal body of the appliance and ends in the earth. So it should never come into contact with live wire. In case the earth pin is connected wrongly with the live socket ,the user touching the appliance might receive an electrical shock.The earthing being thicker in the diameter as compared to the other pins,can never enter the live or neutral socket.Thus the appliance can get connected only in proper electrical position.

Secondly,the earthing pin is made longer than the other pins,so that it gets connected the earth terminal first before the other pins(live and neutral) make the contact in their respective sockets.This ensures safety of the user.

In case there is a short circuit,due to lose contact,weak insulation or electrical leak,as soon as the appliance is plugged in,the current from the electrical appliance flows to the earth without harming the user.Hence it is always safe to install thick high tension wires for earthing.