What is the function of the small circular black plastic at the bottom of the light

These low-pressure gases prevent the filament inside the bulb from burning out; it also relieves some of the stress on the glass globe from normal atmospheric pressure, lessening the chance of glass breakage.

Explanation:

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .