Fatties are bored citizens who have turned to gluttony as a means to pass the time. They are so gigantically fat that they need a belliwheel to prop up their prodigious gut.
In the future, fatties can count as athletes: competitive eating is an international sport, ranging from the Olympics to criminal back-street eating games. Some contests even have fatties eat items like bedspreads and car parts. Mega-City One has fat camps where parents send juves to gain weight  and poor citizens may try to bulk up in the hope that eating games might be their ticket for prosperity.
Fatties were commonplace in the 2100s, when gluttony was one of the few traditional pleasures that weren't illegal. This gave rise to both the coveted Mega-City Fats award and the first one-ton man, Big Lard Ringer (who was swiftly arrested after he rolled downhill).
When food rationing was enforced following the Apocalypse War of 2104, the fatties were left distraught. Begging and fraud spread throughout the community. On the flipside, many norms began assaulting them for being fat when others were starving (and some suggested eating fatties as a compromise). A group of Fatties called the League of Fatties began stealing or looting extra food to keep up their prodigious girth. In order to restore order, the city passed the Fat Control Law: all Fatties were restricted to four special blocks and not allowed out until they were below 300 kg [661.3 lbs]. There they had to submit to a regimen of exercise and a tightly-regulated diet. The fatties that did get out often binged on food to celebrate and ended up back in the blocks the next day.
Illegal eating championships went on for years, with fatties smuggled or broken out of the blocks.
In 2107, rationing was over and the fatties were allowed back into society. Eating championships were legal once more and attracted huge crowds.
The fatties were so common and the eating championships so lucrative that by 2123, thin juves would be sent to fat camps. The heftier juves were astounded to learn 21st century camps had expected them to lose weight.