Imagine a sophisticated young executive mocking a friend because he's dating an African-American. Not likely today, is it? Nor would he exude horrified disbelief if that friend's love-object were gay or disabled. But let's suppose, as Neil LaBute supposes in his latest play, that the woman is fat. Now, there's a prejudice openly alive in an age that expects its females to be inspired, not by Renoir and Rubens, but by Modigliani and the stick-insect culture.

LaBute is often described in interviews as a bear, even a big cuddly bear, and in the programme he makes it clear that he identifies with Helen, the “overweight, sensible and perfectly lovely heroine of Fat Pig” who, like him, is “a stress eater”. But as he would agree, there's a rather obvious difference, which is that she's a woman and more easily mocked than most plus-sized men. Indeed, Kris Marshall's Carter can't disguise his hilarity when his office chum, Robert Webb's Tom, falls for Ella Smith's Helen, who stuffs herself with pizza or whatever else is going.

LaBute is a fine dramatist, and has written a lot of hard-hitting stuff, including the screenplay for In the Company of Men, in which two embittered yuppies try to destroy a deaf secretary by wooing and then dumping her. Fat Pig, though, involves more institutionalised bigotry. The play evokes cruelties so everyday that many wouldn't see them as cruel at all. At any rate, it's unsurprising that Carter should try to talk Tom out of a liaison he thinks he'll regret, and Tom be defensive enough to hide Helen when he takes her to the company's beach party.

Add sassy, dangerously comic dialogue and some subtle observation — the heroine keeps pre-empting attack by badmouthing herself, for instance by saying that she's a Helen who needed a thousand ships to carry her home — and you've a play that's more resonant if less punchy than In the Company of Men. LaBute leaves you feeling there's an awful truth in Carter's overall warning: “People aren't comfortable with difference: fags, retards, cripples, fat people, old folk even.” But the play has problems, principally that Tom and Helen's romance is so sudden you can't believe that it has the depth the plot needs. And one of the four characters, Joanna Page as the trim beauty Tom rejects, is too obviously there to represent female “normality” and spout fattist bitchery.

Page could be harder, tougher, but all else in LaBute's production are excellent: Webb radiating an earnest, flummoxed decency; Marshall lolling, grinning, playing the stirrer and joker, but suddenly serious when he recalls his embarrassingly obese mum; and Smith, at ease and not quite at ease in her own skin. Could LaBute have evoked the “stress” that originally bloated her? Maybe, but it's a touching portrait all the same.

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