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I think a lot of people are making this mistake. Obviously, an obese person (BMI > 30) would be plus-size, but that doesn't mean that a woman who has to wear plus-size clothes is obese.
A BMI up to 25 is still considered having a healthy weight, and there is a wide variety of body shapes that can result in a BMI of 25 and a necessity to wear sizes 12 or higher. So you can wear plus-sized clothes and still be a healthy weight. All this crap about obesity being the problem is a distraction.
Also, as a guy who knows how to use a sewing machine (and is secure enough in his masculinity to say that), there isn't much difference in the amount of fabric used for a size 12 or 14 versus, say, a size 6. That economic excuse is bogus. It could be that the (slightly) larger pieces of fabric needed for larger sizes creates a good deal more waste fabric, because raw fabric comes in a fixed width, and if you can't cut pieces side-by-side on the fabric then you end up with lots of scraps that are (slightly) too small to be used. But they can solve this by using wider bolts of fabric, or by cutting out large sizes and small sizes simultaneously.