In recent years, new players have occasionally entered the game, as the designer Elie Tahari did not long ago; but just as many, or more, have exited or cut back. In 2005, H&M stopped selling plus-size clothes in the United States. J. Crew and, more recently, Ann Taylor began selling plus-size clothes exclusively online. Given the fit challenges a plus-size customer faces, the shift to a virtual space where nothing can be tried on can seem alienating to her — a directive to wear a muumuu. She may not particularly like muumuus, and she doesn’t want to be regarded as someone for whom muumuus are a reasonable choice.

The market for plus-size clothes is effectively a Catch-22: women purchase less than they might because what they see on the racks doesn’t appeal to them; manufacturers and retailers cite poor sales figures as evidence of low demand and retrench, failing to provide the supply that might meet changing tastes. But correcting the imbalance isn’t a simple matter of translating a Milan runway look into a larger size. 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.” A designer must then find a fit model who represents that type and develop a pattern around her. But even within the subcategories, there are levels of differentiation. “Armholes are an issue,” Fasanella told me, by way of example. “If you have decided to go after the woman who is top-heavy, well, some gain weight in their upper arms and some do not. There are so many variables; you never win. It’s like making computers and then deciding you want to make monitors; a monitor is still a computer product, but it’s a whole new kind of engineering.”

The few elite manufacturers who have spun off into the plus-size world prove that capital and patience are required in equal measure. Marina Rinaldi, a branch of Max Mara, sells three million pieces of plus-size clothing each year in 93 countries. To make these high-end plus-size clothes, Marina Rinaldi employs 50 people in the paper-pattern department alone. Three fit models are in the design studio every day. Because most of the fabric is stretch, its tensile strength must be tested: 80 percent is manipulated mechanically or by hand to measure resilience. Cutting a stretch fabric is more complicated, because it doesn’t rest easily on the table; stitching it requires using a yarn with elasticity. By the time a Marina Rinaldi tunic-length, drawstring cardigan arrives for sale at $395, it can seem almost economical.