In 1984 by G. Orwell, there are many people that are described as obese, especially among proles. If the population of Oceania lives in poverty, then how come they have enough food to become overweight?
There's a thriving black market in goods including fattening food (such as chocolate and butter) as well as a general abundance of unhealthy carbohydrates :
The wooden- seated carriage in which he travelled was filled to overflowing by a single enormous family, ranging from a toothless great-grandmother to a month-old baby, going out to spend an afternoon with 'in-laws' in the country, and, as they freely explained to Winston, to get hold of a little blackmarket butter.
Chocolate normally was dullbrown crumbly stuff that tasted, as nearly as one could describe it, like the smoke of a rubbish fire. But at some time or another he had tasted chocolate like the piece she had given him. The first whiff of its scent had stirred up some memory which he could not pin down, but which was powerful and troubling.
Where did you get this stuff?' he said.
'Black market,' she said indifferently.
In the book, there is only one (named) person referred to as being fat; Winston's fellow Ministry worker, Tom Parsons.
Parsons, Winston's fellow-tenant at Victory Mansions, was in fact threading his way across the room — a tubby, middle-sized man with fair hair and a froglike face. At thirty-five he was already putting on rolls of fat at neck and waistline, but his movements were brisk and boyish
Given that Parsons evidently spends much of his time playing table-tennis, hiking and "other physical activity", it can be supposed that his propensity for obesity is as a result of his buying black-market food and exceeding his ration in the same way that Winston spends his excess cash on diaries and fancy pens.
Strivers + Pushers.
Overall, it's clear that poverty and hunger in the world of 1984 are far from universal. Food of all types is readily available, if you know where to look and are willing to pay. All of the people in the book who're overweight are typical "strivers", pushing and shoving their way around. Even the proles are able to get hold of sufficient food to maintain an (un)healthy weight;
Two bloated women, one of them with her hair coming down, had got hold of the same saucepan and were trying to tear it out of one another's hands.
the way was blocked by an enormous prole and an almost equally enormous woman, presumably his wife, who seemed to form an impenetrable wall of flesh.
It was only on very rare occasions that one saw inside the dwelling-places of the Inner Party, or even penetrated into the quarter of the town where they lived. The whole atmosphere of the huge block of flats, the richness and spaciousness of everything, the unfamiliar smells of good food and good tobacco
Later, even a small change in position results in Winston having more money than he can spend, sufficient to allow him to drink himself into a stupor on a pretty regular basis;
As the gin rose in him he belched through purple lips. He had grown fatter since they released him, and had regained his old colour — indeed, more than regained it. His features had thickened, the skin on nose and cheekbones was coarsely red, even the bald scalp was too deep a pink
Out of universe, you don't necessarily need a surfeit of food to be obese.
Some people are genetically predisposed to be overweight [citations needed]
It's basic fact of fitness that whether you lose or gain weight doesn't depend on what you eat nearly as much as on whether your calorie intake is less or more than your calorie expenditure.
As such, someone eating bread sitting on their behind all day is quite likely to get obese, especially if so genetically predisposed, without needing to consume extremely high quantity of food. As another answer noted, the people aren't so poor as to starve and have no access to cheap, unhealthy food.