I have a question. Who wants to play ‘the fat girl’? Even in a comedy. Who reads the script and then auditions for that role? Does she not know? I always wonder. Or genuinely not care? Or did her agent persuade her with the immortal words, “This part is just made for you”?

I raise this now because Fat Pig isn’t really a work of fiction. It’s a two-hour dose of hilarious anthropology administered in Neil LaBute’s brilliantly – often mercilessly – realistic dialogue. And it asks why, of all life’s cruel afflictions, being fat should be so much more burdensome than being ugly, spotty, badly-dressed, or even a train-spotter.

Tom (Nicholas Burns) meets Helen (Katie Kerr) in a crowded restaurant. She’s approachable, generous and funny, with “a potty mouth, but a really cute laugh”. He has the salad. She has the salad. And the pizza. And the garlic bread. And a dessert. Two, actually. There’s a reason why Helen’s a little, ah, heavy set.

Tom is a regular guy: no puritan, certainly, but hardly a libertine. He likes Helen very much, but struggles with the unhesitating condemnation of Carter (a wonderfully vicious Kevin Bishop), the mate/colleague from hell. Meanwhile, he’s still a little caught up with Jeannie (Kelly Brook), office totty par excellence. You see his dilemma.

This isn’t just a string of fat jokes, either (though there are some corkers). Fat Pig is challenging, in the moral sense, rather than the euphemistic, theatre-critic sense. In Tom’s defence we don’t think he’s personally troubled by Helen’s size so much as ruinously agitated by the judgements of others (for observing which, of course, he despises himself all the more). And seriously: find me the straight young man whose pure love wouldn’t be challenged by the sight of Kelly Brook in a bikini and heels (picture please, editors!).

Kevin Bishop says he laughed uproariously on reading his script, then immediately felt guilty. It’s the same for the audience. Like him or loathe him – you won’t be ambivalent – Carter is real, the mischievous voice of unsettling truth over Everyman’s shoulder: “You laugh at the same jokes and look at the same asses that I do.” So any man determined to argue that Jeannie and Helen (more comfortable if we stick to character names, huh?) are really equally beautiful ‘in their own ways’ will need his poker face when, in the play’s closing scene, LaBute puts the two women on stage together for the first time – in swimwear.

Two points in defence of my fellow men, lest anyone kid themselves this is a play about how shallow we are. First, the women in the crowd absolutely loved Carter (WARNING TO MEN: this is a trick – do not try any of his lines at home!).

Second, of all the bitching in the show, the most unabashed comes from Jeannie. And, for that matter, from this week’s arm-candy: “Kelly Brook’s got weird boobs . . . but not in a bitchy way.” No, course not. Still, I offer it by way of balance.

n Fat Pig is at the Comedy Theatre, Panton St, London, SW1. Tickets: £20-£45 (www.fatpigtheplay.com).