Giantess is a permutation on BDSM primarily, a power-top to the maximum. It often features men or women used as sexual toys for the giant in question, or in more extreme fetishes, as a food source, but both are in the end the same: all about reducing the secondary party into an object to degrade and use, while cementing that the "top", dominant, party is unassailable and unimpeachable from their position of power over others. To reach that level of fantasy domination outside of the giant(ess) genre usually requires some extreme magic-based slavery element in the worldbuilding, which sometimes isn't to the taste of the reader.

It is a sister fetish to the "tiny" fetish: fairies/imps/pixies/etc, all the things less than a foot tall, which accomplishes the "giant(ess)" element without the larger party needing some kind of in-universe explanation as to how they can surpass the square-cube law.

Oddly enough, though, the "tiny" element is more often Male Dom/Female Sub, while the "giant" element is more often Female Dom/Male Sub, from my limited exposure (so at least for the most commonly posted stuff on 4chan or the like). This might relate to the "safe horny" theory posited by a poster above.