Ah...
but I can see where you're coming from. More material should = more money right? It happens with commissions - the more material and time needed the more you will be charged.
The thought may seem offensive, but when you think about it it is slightly unfair that you pay the same amount for less material. Eg you have to pay $50 for a 10ml perfume while others are able to pay $50 for a 100ml bottle of the exact same perfume from the same store = unfair.
However, making everything different prices would be confusing for the company etc and it's easier (though unfair) to price everything the same.
I don't think they should make the bigger sizes more expensive (because it'll be hard to afford after a while), but make the smaller sizes less expensive instead.
I have nothing against bigger people, but you should be charged for the amount of material used. It's the same with everything - clothes, shoes, makeup, accessories, food etc.
this seems the only person to reply sensibly.