As you approach the old Baltic flour mill in Gateshead, now the city's major space for contemporary art, you are greeted by an unusual sight - an oversized and dumpy detached house sitting forlornly on the public walkway next to the River Tyne. This bulging abode, which seems to be oozing out from under its own roof, is the Austrian artist Erwin Wurm's sculpture Fat House, one of a series of voluminous works he has made that includes a flabby Porsche (Fat Car), a body-mass-enhancing fat suit, and even a calorific recipe book, From Size to Size XXL, with instructions on how to gain two extra sizes in eight days. Inside the gallery, Wurm's exhibition includes life-size figures of the artist horribly bloated after apparently "swallowing the world" and a stuffed sausage dog that appears to have ingested a whole brick.
I interviewed Wurm soon after he had finished his first Fat Car and he assured me that he had no ambitions to "make the whole world fat". But the world is getting fatter, whether we like it or not. As waistlines bulge and obesity levels continue to rise, art seems to be fattening up, too. Of course, this is not entirely new: Lucian Freud has made a career out of depicting rolls of naked flesh, often using oversized models such as "Big Sue" Tilley and the larger-than-life cabaret performer and artist Leigh Bowery. Jenny Saville represents the next generation of figurative painters and her billboard-sized portraits, self-portraits and corporal landscapes render herself and others either as implausibly overweight or as slabs of immobile meat.
Other artists whose work explores excess fat are the late Duane Hanson, with his comic-ally obese waxworks of American tourists; the photographer John Coplans (died 2003), with his startlingly naked images of his own ageing folds and hairy mounds; and the invasive plastic surgery performances of the French artist Orlan, after which bits of her skin and fat are sold as quasi-religious "relics". These images of obesity are generally negative - violent, off-putting or grotesque. The corpulent body has become artistic shorthand for our flabby consumer lifestyle.
It was not always so. Some of the earliest known sculptures - such as the magnificently rotund Venus of Willendorf, which dates back to roughly 24,000BC, and the Venus of Berekhat Ram, which is 230,000 years old - are lovingly carved stone figurines of female torsos with exaggerated breasts, buttocks and bellies. Early man is believed to have crafted these figures as symbols of fertility or good luck.
Plumpness was preferred in the art and society of the 1600s, when Peter Paul Rubens painted generously proportioned ladies, and before the 1890s, as seen in Pierre-Auguste Renoir's voluptuous nudes. But changing fashions in the 20th century produced seismic shifts in our body image, to the point where the only artist since the 1960s to have depicted fat people consistently in a sensual, Rubensesque manner - Colombia's most successful painter, Fernando Botero - has been ridiculed by the critics.
Despite this historical shift away from a corpulent self-image, art itself is getting bigger all the time. Certainly Wurm's Fat House is not just an indictment of consumerism: it also pokes fun at the obsession for commissioning ever larger public sculptures to plonk in our parks and plazas. In Italy, this kind of boasting ("My monument is bigger and better than yours") is known as campanilismo, referring to the pride with which some Italians defend their local bell tower or campanile. For some reason, such regional one-upmanship is particularly strong in the north of England: in recent years Manchester's B of the Bang, by the designer Thomas Heatherwick, has become Britain's largest public work of art, snatching the honour from the towering Angel of the North, a few miles from Fat House.
Antony Gormley's Angel may not itself be an image of obesity, but it is a symbol of a trend for overblown artistic statements commissioned by puffed-up local-government councillors. More often than not, the results of this "Angel effect" are blots on the landscape. We can only hope that some of the ridiculous structures in the pipe-line for 2007 - such as the 190ft Solar Pyramid sundial near Chesterfield and Charles Jencks's 1,500ft-long "Goddess of the North", to be carved out of a Northumberland slag heap - will be less hopeless than they sound on paper.
The nation's galleries are also growing (Tate Modern and the Baltic have led the way in super-sized art venues) but their bellies are not getting any fuller. Contemporary artworks are increasingly beyond the paltry budgets of most museums and art institutions, thanks to a bloated international market fuelled by mega-rich patrons and private collectors. There is a great painting presaging this overstuffed art world. The Patron (1997), by the American artist Ashley Bickerton, shows a slobby, half-naked collector sitting on a chair, one hand down his Y-fronts and idly flicking TV channels, seemingly bored by the Mondrian and Brancusi artworks around him. The buyer of this work? Charles Saatchi, naturally.
There seems no end in sight for gigantism in art production. Unless the bubble of the market bursts, commercial greed will continue to swell the coffers and squeeze the creative arteries. But "fat", applied to depiction of the human body, seems destined to remain a dirty word.
"Erwin Wurm" is at the Baltic Centre for Contemporary Art, Gateshead NE8, until 10 October. For further details call: 0191 478 1810. www.balticmill.com