Anyone who’s ever bought clothing they’ve seen a model or a mannequin wearing has probably had that moment, when you get the item in your non-mannequin size, put it on, and… it just doesn’t fit well, now that it’s on a real person. Why does this happen?

Of course, there’s the simple reason that not every woman has the exact same proportions, but Quartz explains that the way brands take their designs from one size and scale it to create those same garments in other sizes can contribute to overly boxy blouses and ill-fitting jeans.

Brands use fit models to size their clothing up or down, choosing people with bodies that have ideal proportions for a standard size 6, for example. These models can help designers see how clothes look on an actual person when they’re walking or moving, and the human mannequins can offer helpful direct feedback as well, like, “This clings weirdly when I sit down.”

Clothing makers then use a process called grading to scale the dimensions of each item up or down, adding an inch to the bust of shirt here or more length to a pair of jeans there. The process isn’t uniform across the industry, of course.

This works okay when the sizes are closer to that of the fit model’s, but can lend a “distorted” or “stretched” factor in larger sizes, one fashion insider explained to Quartz. After all, human bodies don’t scale up uniformly in nature.

So what can brands do to fix this? For a start, hire larger fit models, instead of just using one model for every item the label offers, as many clothing makers do.

“Fit regularly on a professional fit model who is a size 18 or the size most sold by the brand, and conduct wear tests and/or size sets,” Dale Noelle, the founder and president of True Model Management, a modeling agency that works with a large number of fit models, told Quartz.

This isn’t as easy as it sounds, however, as it costs time and money to use separate fit models for each garment of clothing. Some labels likely either don’t want to spend that dough, however, or don’t realize that there’s a reason to: If brands can’t sell badly fitting clothing in larger sizes, perhaps they decide they don’t have customers who want larger sizes, so they don’t design new products with them in mind

This is a problem for many shoppers, as Quartz notes that the average American woman is around a size 16 or 18 today. What does this mean? Brands could be missing out on a lot of sales, sales they could make if they invested in their fitting process.

One analyst for research firm NPD Group estimates that sales of plus-sized clothing — usually size 16 and up — are only half what they could be if there were only more options out there for shoppers.

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