After playing around with it for a bit tonight, I got this (which isn’t all of it, but still)
One day, the Earth is conquered by highly advanced, giant, hugely muscular, super strong alien women. This is a shock. For a while, the humans are dominated.
Then they start transforming women they consider worthy into more of them. Men fight back. They're killed or captured in great number. So much so, that they have no choice but to surrender completely.
Men not involved in the rebellion are spared and mostly allowed to live their lives. Some of them have wives who were chosen to undergo the transformation. These women are in a difficult position. They still want to have sex with their husbands, but they now have super human abilities and can easily crush men without breaking a sweat.
Lesbian sex is allowed and encouraged under the alien rule, but sex with men is still necessary for reproduction, so the aliens long ago invented medical nanites that repair the male body as needed. The nanites also feed on testosterone, so men now produce far less of the stuff than women.
Within a few generations humanity largely comes to accept that this is how things are. Of course there are always the fringe groups that hate the transformation and the aliens with a passion, but there's not much they can do.
The world is eventually united as the Empire.
Human women compete annually in a series of athletic and intellectual tests for a chance to undergo the transformation into a giantess.
The transformation process is quite painful, not to mention dangerous. Fully grown female giants can be quite imposing, and while they suffer none of the drawbacks of regular giantism, their height makes them incredibly strong.
On top of that, even in proportion to their height, they're far more muscular and stronger than any man ever could be, even before the aliens came. Large breasts and wide hips are part of the package as well.
The transformation process is also potentially life-threatening. It happens very rarely, if the candidate survives it, she grows to be a giantess.
On average the growth takes about a week, and the subject grows to between 30 and 40 feet tall. In addition to the great strength that comes with growth, she also gains the ability to withstand great amounts of damage.
For obvious reasons the aliens never had a nudity taboo, and this rubs off on humanity, especially the transformed women. They're completely comfortable with their bodies and don't feel the least bit embarrassed about being naked.
Giantesses, of both human and alien origin, continue to slowly grow and gain muscle all their lives, which due to the Empire's advanced medical technology can last centuries. Many become professional gladiators and fighters, with great success.
Wrestling and general contests of strength are the most popular sports among the giantesses. They also participate in a wide variety of other contests, from javelin to swordplay.
Many giants marry and have giant children, though it is not uncommon for a giantess to have several husbands given the chance.
It is also accepted for women (giantess or otherwise) to have sex with other women, even outside of marriage.
Giantess pregnancies are very dangerous and subject to greater than normal miscarriage or stillborn rates, but the babies that are born are indeed gigantic.
They are exclusively female, and eventually grow to a similar size as their mothers. While the rate of growth is much slower, it's still a very large size.
Space travel has become far more convenient and accessible, although only a small percentage of the galaxy has been explored. The only other intelligent species known to the Empire are the Kirath, who have been nomadic for thousands of years since fleeing their inhospitable homeworld. The Kirath live in extended family groups, alliances of which form large space fleets called Tra-Ganth, which means "Void Refuge" in the Kirath language.
The Empire has an official policy of "gerontocracy", or rule by elders. All higher civil and military positions in the Empire are held by those over the age of twenty.
