If you wanted to build a shed 8' by 12', you'd have to buy a certain amount of lumber, nails, etc. If you wanted it to be 10' by 12', you'd have to buy more of those materials, ergo, it would cost you more.

We cannot expect lingerie companies to subsidise our larger sized breasts by writing off some of their costs. Well we could, I guess, if they were using larger sizes as a "loss leader" but I really don't think that's going to happen!