It is not because Miuccia Prada cannot abide women who are a size 18 that she makes no dresses in size 18. Matters of image and fears of brand diminishment may play a role, but the business of making plus-size clothes turns out to be enormously complicated.
The most formidable obstacle lies in creating a prototype. If you already have a line of clothing and a set system of sizing, you cannot simply make bigger sizes. You need whole new systems of pattern-making. “The proportions of the body change as you gain weight, but for women within a certain range of size, there is a predictability to how much, born out by research dating to the 1560s,” explained Kathleen Fasanella, who has made patterns for women’s coats and jackets for three decades. “We know pretty well what a size 6 woman will look like if she edges up to a 10; her bustline might increase an inch,” Fasanella said. “But if a woman goes from a size 16 to a 20, you just can’t say with any certainty how her dimensions will change.”
Thin people are more like one another; heavier people are less like one another. With more weight comes more variation. “You’ll have some people who gain weight entirely in their trunk, some people who will gain it in their hips,” Fasanella continued. “As someone getting into plus-size, you can either make clothing that is shapeless and avoid the question altogether or target a segment of the market that, let’s say, favors a woman who gets larger in the hip. You really have to narrow down your customer.”
--Gina Bellafante, NYT Magazine, on Anna Karenina's adage adapted to body weight