Much is being made of the size 000 offerings  recently introduced to J.Crew. Yes, the number sounds extreme, but in the scheme of things that, or really any digit, is fairly irrelevant when you get down to it.
That's because by and large a size 00 — or a size 22, for that matter — means different things across the board, as labels determine measurements at their own discretions. So a 6 at Topshop may fit you flawlessly, but Madewell's 4 could be too big, and vice versa — and it all begins with a fit model.
Nothing like the runway models or print models we often see on the pages of magazines, these are women selected by brands to represent their average consumer and ensure the clothing fits as such. So we sat down with Nicole Callahan, a former fit model and current designer at sportswear brand Lyssé  — because no fabric knows a good fit like spandex — to discover all the secret ins and outs that go into sizing our clothing.
Source: Shutterstock 
POPSUGAR: What is a fit model?
Nicole Callahan: There are actually two types of internal models: fit and showroom models. When we have presentations for buyers and need to show them what a product looks like on the body, we have a reasonably proportioned showroom model come in. Fit models are internal — you only use them with your design team. So you start with an idea for a sample, you get it back from the manufacturer, and you put it on the fit model to see how it works.
PS: How do brands choose their fit model?
NC: They're the type of person each company identifies as their own specs. There's a 4, 6, 8, 12 and small, medium, large — well, what does that mean? At one store you're a 6, but at another you're a 4; the reason for that is each company builds its own standards for who they think their ideal woman is. Some higher-end companies, for example, cut to a smaller woman in general. You choose your fit model based on who you think your consumer is.
PS: How many fit models do brands have?
NC: It's getting more sophisticated — it used to be that you would have one size fit model. Maybe you would pick your medium, usually your 6. What's happening now, is that companies are checking the fit on a number of different sizes. Bigger companies have done that for years, but more and more people are doing it.
PS: How many models do you use?
NC: I have three. I’m fitting on a size small and checking it on a small model. I actually start my fit process at a size small, and then I grade it up to my medium model and see what works. Then I know how my large and my extra large will fit. And then I actually have a plus-size model as well.
PS: You can't just grade up your medium to plus size?
NC: You shouldn't — if you do, you're going to be in trouble, because women's bodies really change by the time they get beyond 1X, which is like an 18.
PS: Do fit models specialize by body part?
NC: Intimate models are their own thing. If you're a bra fit model, then you're a bra fit model. Clothing's a little different. When I was a fit model, I mostly did denim, and you do find people gravitating toward certain groups of things and get streamlined.
PS: So if a company changes fit models, do they replace them with one who has the exact same proportions?
NC: It's a lot harder than it seems, because as much you can look for the perfect shaped body, there's no such a thing as a standard body that hits all the "average" measurements. For example, every woman has hips that are a little different — one woman could be wider, another has a larger behind — and they could still have the same measurements.
Source: Instagram user lyssefashion 
PS: Are there supermodels in the industry?
NC: Kind of! I just started working with a petite model who has been working for 30 years — and she clearly does everybody's petites.
PS: Has she had to keep the exact same measurements for 30 years?
NC: Yeah. They all have to go in for measurements, same as all models. The ones who work for agencies have to go in once a month to make sure they’re keeping their measurements.
PS: Are there any trends in the industry?
NC: There's one trend that has been going on for a long time, and it’s called vanity sizing. Vanity sizing is what everyone has done in the last eight years to accommodate women (especially in America) who are bigger. So what was once a 6 became a 4, a 2 became a 0, across the board in the industry.
PS: Would labels keep the same model and just change the size?
NC: In some cases, yes; people aren't fitting on a different-size body than they had been before.
PS: Does that mean there is no standard for sizing?
NC: There are certain measurements that are known to be true; there is a size small for the industry. Whether you call that a 6 or an 8 is where it gets fuzzy. What people maybe used to call an 8, they’re calling a 6.
PS: Are those measurements you mentioned where brands get their sizing guides?
NC: That's where the numbers come from. When brands list an inch range, that's based on their fit model. I build my size scale based our my medium fit model's measurements, and she's an 8 to 10. So I write up a range that is based on her chest, waist, hips, legs, whatever. And I do the same for 1X, and build out the inches in between (subtracting for smaller sizes and adding for larger sizes). And the numbers I come up with will probably work for smalls and larges.
Source: Instagram user lyssefashion