If you buy children's clothes, you'll have probably noticed that you have to pay a good few quid more for a T-shirt in age 12 than you would for the exact same T-shirt in age 5. Fair enough, the bigger size uses a lot more fabric.
Why then, is a size 24 ladies' T-shirt the same price as the same T-shirt in a size 6, despite it using a lot more - maybe even twice as much - fabric?
Would people be more motivated to stay a healthy weight if they had to pay more for bigger clothes? Why don't we charge more for bigger clothes? Are the prices based on the average amount of fabric used - so skinny people pay more and very large people pay less than they should do based on the resources used to make their clothes?
FWIW I'm marginally overweight and wouldn't have a problem paying more for my size 14 than a 6. I think it would motivate me to lose some weight if the size 10 was cheaper though!