The story behind this, as bein' worked on by me and  GeckoDude92 is that a group of giantesses had held over a territory with their giant influence, yet while they may have their fun on the tinies, they are ultimately gentle enough... however, they decide to get themselves a f**kbuddy or a man who can take them without breaking. After many a trial and choosing and proving his worth, a particular young man gets to be chosen as their shared mate. Of course, he does need to be uplifted some.

However, whether intentionallly or not, he ended up growing even more than expected and now they indeed got a huge man able to handle them all sexually... Not that the tinies end up noticing much besides the giantesses being more mellow nowadays, even if back in their temple things may have changed some. The leader of the giantesses is still in charge nominally, even if even she can't come up to his pecs... and she doesn't mind. Especially since he's not interested in taking over, as he gets to bang the giantesses and even served by them, at least the ones who prefer his company in the temple. Other giantesses mostly still have their fun mainly with tinies, but they do find him a fun change too...

This setting may yet involve an inverted case in some other territory where a group of giants chose a lucky girl to ascend up amongs them in their temple... and ended up getting a benevolvent but equally horny matriarch instead, but that's for another time.

This is rough but its a start.