Why earth pin in plug is larger than the other pin?
Generally the ground pin is a bit longer and can carry more current than the other pins because it's more important in case of a failure of insulation in the device for the ground connection to remain connected long enough for the fuse or circuit breaker to open the circuit. 

It's also typically longer so it makes contact first and you cannot have a situation with hot connected and no ground. Of course in normal operation it carries no current. 

Sometimes, particularly when the ground is not there, the other pins are asymmetrical so that the hot and neutral cannot be easily swapped. I don't believe that applies to standard Indian plugs, which are a triangle arrangement of round pins, similar to some South African plugs.  

Images from here: 



As Jasen points out, in every design of mains plug I've seen the size and shape of the ground plug is such that it cannot be inserted into either of the other holes. For example, the British BS 1363 plug has a ground pin that is about 8mm wide vs 6.5mm for the power pins.