Claudia Pritchard: The more you eat, the more you pay...

For travellers flying with hand luggage only, the checklist is simple: passport, ticket, money, five layers of clothing, one on top of the other. Three necklaces, two belts, four scarves, two cardies, one worn properly, the other tied round the neck, like a shop-window mannequin. Sandwiches in this coat pocket, fruit in the other, newspapers in the left hand, novel, guidebook and dictionary in the right. Wellingtons or walking boots, hat, binoculars. Teeny, weeny little case on wheels, containing virtually nothing.

Size and weight restrictions on cabin bags mean that nearly everything needed on the trip must be pocketed or worn. It can be done, but the business is hot and cumbersome. And yet, when I take these elaborate precautions not to exceed 10kg with my 55cmx40cmx20cm bag, and a supplement to pay, chances are the person behind me in the queue for security will be heavier than me, their rolls of fat waved through unchallenged, while I sweat along ahead of them, wearing a small branch of Gap.

Air France's proposal last week to make obese travellers book two seats is, on the face of it, discriminatory. It sends the questionable message that those who do not fit a template of physical perfection should be penalised. This is not a notion any decent person would subscribe to in daily life. Blessed are the plain, the knock-kneed and the pock-marked, for they shall develop fascinating personalities, because they cannot otherwise stop traffic with what they were born with or grew into. But to be massively overweight is, in the vast majority of cases, a life choice. Only a very few, very unfortunate people have a medical condition that causes excessive weight gain, and clearly they should be exempt from any new ruling.

No one is born weighing 17 stone, and no one can blame their parents for ever, even if they were force-fed Jaffa cakes in the ancestral home. Adults are responsible for their own calorific intake, and if they consistently order the spotted dick and drive to the cinema they will be fatter than the person who has the fresh fruit salad and walks to the gym. You do not put on weight if you so much as look at a cream cake: you put on weight if you eat it and do not burn off the 425 calories it contains.

The National Health Service, pension plans and the life assurance industry are watching our weight, even if we are not, and they all know that the cost of obesity, as of smoking or drinking alcohol to excess, is hefty. Some bright spark will point to the jogger whose collapse mid-training has put a resus unit on red alert, but it is being overweight that exacerbates many life-threatening conditions, and it is not body fascism to wish people would stop eating themselves to death.

And while they live their compromised lives to the full, jetting hither and yon, would airlines have a case if they put a weight limit on passengers? Yes, because this is about fair shares. There is a finite amount of space on a plane, and, Byzantine fare structures apart, each passenger pays for a precise allocation of that space. There is no more excuse for encroaching on your neighbour's than there is for eating their coq au vin and strawberry tartlet.

The Air France scheme may never get off the ground with other airlines, and anyway, deciding on a "right" or "wrong" weight for passengers is like deciding whether blue is a good colour. It all depends.... The carrier says that its proposal is a recommendation only, and that it has been around since 2005. A modification, from next month, that the extra fee will be refunded if the plane is not full, is what has brought it into the headlines.

But it has also attracted attention five years on because we are developing an ever more acute sense of what is just or unjust when we share out the goods. How fair it is on the rest of the planet for any of us, fat or thin, to fly at all, is another issue altogether.