Earthing pin

The Earth pin in the UK isn't round, it's rectangular..but it too is bigger than the L or N pin

But my guess is so that you can't stick it in the Live or Neutral hole!

I guess you are talking about BS4343 type plugs not BS1363 type square pins, but than reason is the same.

The earth pin and wire is the most important wire in any piece of Class 1 portable equipment, traditionally in the UK the earth pin was larger to ensure a low impedance path and therefore had a larger cross sectional area.

The earth pin also traditionally operated the socket shutter mechanism which guarded the Live sockets when they were unused and therefore require some additional mechanical strength.

In all UK power plugs the earth pin is always longer than the other pins in order to ensure it was the first pin to connect and the last pin to disconnect, thereby ensuring that the equipment was earthed before and after the Live pins made connection/disconnected.

The reason for this was there were no switches on earth AC power sockets and the plugs were inserted and withdrawn while the socket was Live

In the case of the BS4343 arrangement the pins are completely encased in the plug body and therefore cannot be touched at all during connection. This is the reason why they are used for outside and damp conditions.

This is not the same with every country some only have two pins and no earth pin apart from a thin scraping contact strip on the outside of the plug.

http://en.wikipedia.org/wiki/13-amp_plug

....so it won't go in the live or the neutral hole, and so that its current-carrying capacity exceeds that of either the live or the neutral.