Why is the upper hole in a plug point bigger than the rest?
I suppose since the earth prong is also longer on these BS 546 plugs, if you could partially plug it in the wrong way (say earth to phase), then someone else touching the appliance at the same time could receive a shock. So the earth prong is thicker so that you cannot plug it in even a little bit in the phase/neutral holes. (Rotational asymmetry would prevent you from fully plugging it in the wrong way, but rotational asymmetry alone won't prevent a partial but incorrectly rotated/oriented insertion when one prong is longer than the other two.)



The same design idea is adopted by the newer UK plugs (BS 1363) that use rectangular prongs: the earth prong in these is again both longer and thicker than the other two.



The reason why the earth prong is longer is more obvious: so that you always make the earth connection before phase is applied (and vice-versa when you unplug it.)