Generations ago we started moving people to the Moon after crucial minerals were found there in vast quantities. Miners now live in sprawling colonies with their families and 1,000s of support staff. Thanks to generous pay and powerful unions (hey, it's science fiction lol) these have evolved into highly prosperous city-states. Meanwhile 100+ years of war and environmental degradation has made life extremely hard back on Earth, and living on the Moon is the dream for billions.
While most of Earthbound humanity struggles to survive, extreme fatness has become a mark of status among wives, daughters, and other Moon women. It means they can afford endless food, don't have to work and can pay to have others perform even the most basic tasks — bathing, dressing, etc. This was enabled by advances in medical science as well as extremely low gravity that allows women to be healthy and mobile at well over 1,000 lbs.
So while their distant ancestors of the late 20th/early 21st century were desperate to be slim, now the competition is to see how fat they can get. There are even illicit drugs that lower metabolism and enable weight gain. All this has led to a highly decadent and hedonistic mini-society of elite superfats that's both envied and scorned by people on Earth, and a small cult of ascetic Moon people who resent them as well for religious reasons.
One day an outlaw mining company, greedy for an advantage over their competition, starts excavating in an off-limits area. They accidentally expose a vast underground cache of a previously unknown element, which sets off a chain reaction that will blow up the entire Moon within weeks. The mining colonies must be evacuated immediately, which means everyone is headed back to Earth.
Now a huge population who've known only Moon living will have to acclimate to Earth's gravity — 6X that of the Moon. This is obviously a special concern for the hundreds of immensely fat female elites, many of whom are so obese and out of shape they won't survive at all. The only hope for them are a precious few spots on orbiting space stations, but these aren't set up for long-term living; it will only buy them time to lose weight and develop their underused muscles so they can hopefully make the transition to terrestrial existence.
This sets up a desperate last-minute competition among elite Moon women, where they must convince the ministry in charge that they're good candidates, proving they have the discipline to diet and exercise — something they've never had to do. There's also a small group of economic super-elites who are trying to usurp a space station for permanent living, so they can continue their lifestyle indefinitely. In the meantime the ascetics are resentful that the fatties are being given any deference, and are actively trying to scuttle the re-integration program and force them to meet their deserved fate.