I never understood the reason behind so many different sizes of plugs. But again I never understood so many TV standards (PAL / NTSC)
Sizes of plugs? It goes back to the time when electricity was invented. The US had a huge supply of copper from mines as far as south America. Europe, not so much. So the US could afford to go with safer 110 Volts at destination (resulting in twice the copper thickness for transmission wires) and Europe went with 220 Volts resulting in half that thickness for transmission wires. So safety being a concern at 220 Volts that heavy-duty snap-in style connector plug is necessary because 220 Volts *will* fry (not only you but) any improper insulation (distance) between the plug pins. The flimsy US connector was fine for 110 Volts.
Regarding PAL/NTSC: Europe was/is metric-centric. The electric clock was one of the first major applications of the electric motor. The US thought it would make sense to make the clock motor (windings) at 60Hz (60 seconds etc) and Europe stuck to 50Hz. Little did they realize that, by using differing standards, they were setting the stage for PAL and NTSC; NTSC is generally used in countries with a utility frequency of 60 Hz and PAL in countries with 50 Hz, although there are now many exceptions. This has to do with alternating current in artificial light (tv studios) meets fps in video cameras (phasing) and maybe even cathode ray tube display operation. PAL also came after NTSC and it set standards to improve color handling and other improvements were added - (res was 20% greater than NTSC, conversion was easier etc.)
That's the gist of it. It began with Solomon R Guggenheim's dad Meyer in the mid 1800's and his fascination with metals and eventually mining and processing. He started out pushing a pram around Philadelphia collecting bits of scrap copper etc. Then he bought up mines, started smelting and when electricity hit - ka-ching!