They aren’t always. Some stores will charge extra for plus size clothing. However, fabric is fairly cheap, especially at the scale of a company that makes hundreds of thousands of the same shirts so its really negligible. Also the process to make the shirt is the same, the pieces that get sewn together are just larger. There may be less profit on those larger shirts, but it’s probably 50 cents at most and it’s not worth it at that point.
There are two major concepts to understand here.
First, material is a vanishingly small proportion of the cost to get clothing to consumers in modern, western countries. Cloth can be woven by machines and there are just as many seams to be stitched in a larger piece of clothing. Instead the big costs are labor, transportation, marketing, and just the space it takes up in stores waiting to be bought. These things don’t really change with the sizing of the clothing so the overall cost to produce remains fairly constant.
Second, the underlying idea that the cost to produce the good determines the price it is sold at is incorrect. Obviously a company wouldn’t sell an item it cost more to produce than it could sell for, but beyond that barrier a company isn’t basing the price on production costs. Instead they are trying to balance the volume sold with the price to maximize the overall profit. Even if something is cheap to produce if they can sell it for a large amount they will. In modern western countries clothing is generally sold based on style and fashion rather than competing for the absolute lowest price possible.
The actual price of material and even production is a tiny percentage of the price the clothing is sold for.
The reason clothing is the same price no matter the size is because people can only buy clothes that fit them, so it would seem pretty unfair to charge people more just because they’re bigger. If a clothing store did this it would be easy for a rival to say that their clothing is the same price no matter the size and win over some customers.
It’s not just fat people. I need larger sizes because I’m quite tall. If a shop was charging more for clothes for tall people, I would not shop there.
Markups on clothes are quite high. A generic pullover tee shirt with a logo on it that sells for $25 dollars might actually only cos $1 or 2$ as it sits on the shelf, including shipping.
So when your markups are this high, it doesn’t matter that the amount of material varies substantially between garment sizes.
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