0Point 1: For case for ground pin. Also ground pin is larger that other two pins so that it would be the first to go into the hole and the last to come out of the hole just for sake of safety
Point 2: This is done to prevent the user from accidentally inserting the live or neutral pin into the ground pin.
Point 3: It is because of simple thing:
i. If all are same sized then you will have 3 choices to plug in but every time you will not make it right so you get confusion.
ii. In order to eliminate that one of them is bigger than remaining ones so that every time you plugin you will get supply even if you use 2 pin plug.
Point 4: R proportional to L/A. So larger the cross sectional area lesser the resistance! So with less resistance in case of fault all the current will pass through that pin(which is the ground) and make no harm to your circuit! Basically its for electrical safety!
