They need some sort of special crystal as well, which are very rare (possible lie) and strictly controlled by the bitch queen, additionally those crystals can blow your head off after some use, literally.
As there is no magic, lets assume the crystal allows the fairies to bend light around them in a tiny bubble, or something like a skintight suit, I choose to prefer the latter, for obvious reasons.
Now, lets assume there were some even more rare crystal, that lets a fairy change her size to that of a normal human. She then would overheat within seconds, then die because she has been cooked alive, followed by a possible explosion. Not sexy.
watch?v=MUWUHf-rzk