So, London's teenagers are slicing each other to bits, the ice caps are melting, inflation is rising and food prices are so high that we'll soon be forced to eat our home repossession orders. And what do thousands of angry British people join an action group to protest about? That's right. The “injustice” of Marks & Spencer adding £2 to the price of an extra-large bra.

This is a Very Serious Matter, apparently. Busts 4 Justice, the name of a Facebook movement, calls it a “criminally unfair” penalty against the fuller-figured woman. It discriminates on the ground of size. It's a “mammary tax”.

What does the existence of this campaign group tell us about modern Britain? Certainly that people can't have all that much to worry about if they've got time to demand rights for DD clackers. Also that most people who shop at M&S are habitual moaners (first the clothes were too frumpy, then they were too raunchy and now the M&S annual meeting has had to be moved to 2pm because elderly shareholders whinged that if it were held in the morning they couldn't use their free travel passes). But mostly it serves to remind us how larger people have actually had an easy ride all these years. Why on earth shouldn't the bigger-knockered pay more? Why should their thru'penny bits be subsidised?

If I go into a shop and buy a large loaf it costs more than a small one. Fact. Just as if I buy a pint of lager rather than a half. Or a four-man tent rather than a two-man. More materials needed, see. It's like those simple sums that you did at primary school on your fingers when you weren't sticking them up your nose.

But if a size 22 person buys, say, a cashmere jumper in M&S he or she pays exactly the same price as someone who buys a size 8 despite the size 22 using up double the amount of very expensive wool. Where's the justice in that, eh? M&S argues, rightly, that for cup sizes DD to G bras have to be made in a different way, they need more components, stronger straps and quite possibly the use of a forklift truck. It inverts basic economic logic not to charge big people more for their clothes.

I have nothing against big people (except when they won't move to the side on escalators) and I'm certainly not against big breasts. I used to have some myself until age and breastfeeding did their thing. And I know what you are thinking - you don't have to be fat to have a large chest. True. But if you are skinny and have big berthas you should not be wittering on about M&S bra prices, but on your knees thanking God that you are blessed. Blessed, do you hear? If you have implants, that's a lifestyle choice and if you are big chested because you are fat - well that is too. Who says so? David Cameron actually, the man who might be our next Prime Minister. In a fit of political incorrectness he suggested that fat people have only themselves to blame (he also applied the same logic to poverty, which is so ludicrous that it warrants a column all of its own).

“We talk about people being at risk of obesity instead of talking about people who eat too much and take too little exercise,” he said. “It's as if these things, obesity, alcohol abuse, drug addiction, are purely external events like a plague or bad weather.”

Obviously he ignores here the genetic component that makes a small percentage of people overweight and that some suffer from illnesses that affect their metabolism. But generally it was brave to say that if you live on buckets of KFC and two-litre bottles of Tizer, don't play the victim when you look in the mirror. If we are to discuss this issue honestly, we should acknowledge that frequently it is not the obese who are penalised in the market-place but the very thin. A 20st (127kg) man can walk on to an aircraft unchecked, but if the 7st weakling behind him tries to check in a suitcase weighing more than 15kg he pays a supplement.

Why? Weight is weight, however distributed. It takes as much fuel to lift a tonne of human fat as it does a tonne of suitcase. The fat and the thin pay the same ticket price, but it's the skinny minnie who gets no share of the armrest. Ricky Gervais (no will-o'-the-wisp) has been taunting his American audiences this week with just this point.

What if airlines charged for tickets according to total weight checked in - ie, passenger plus baggage? Big people would either pay more to fly (only one US airline, Southwest, implements this reasoning) or take no clothes on holiday. Since men generally weigh more it would give rise to women-only airlines, which at least would give the trolley dollies a break from people trying to look up their skirts. But we know it won't happen. So a two-year-old who weighs about the same as a Twix wrapper continues to pay the same for a seat as the 20st Mr Blobby whose thighs block the emergency exits. I'd call that positive discrimination.

Beckie Williams, the founder of Busts 4 Justice, who'll no doubt be offered a modelling contract any day now, argues: “If you don't charge a size 20 woman more for a pair of trousers, then why should you pay more for an E cup bra? I have been a size 8 all my adult life, but have never benefited from smaller-priced pants.” But that's just it, Beckie, bigger bras should cost more, small pants should cost less. Inadvertently, you have made a better point.

Many of the internet discussions that have sprung from this issue suggest that there are plenty who agree. One contributor may not even be joking when he suggests that if you charge enormous people more for clothes and travel then they'll walk more, have less money for food and the obesity crisis will be solved overnight!

It's a harsh idea, but perhaps one that David Cameron should consider for his manifesto. In his let's-call-a-spade-a-spade politics, he might just find that it has got legs.

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