Why the same size fits differently at every brand

You have not changed shape. The clothes have.

A woman's size 12 in 1970 corresponds to roughly a size 6 today at many retailers. A men's 32-inch waist frequently measures 34 or 35 inches on the garment. Neither is an error. Both are the accumulated result of decades of small decisions, each individually rational.

Where sizes come from

There is no legal or regulatory definition of a clothing size in the UK, the US or the EU. There are voluntary standards, and they are voluntary in the fullest sense — almost nobody follows them precisely and there is no consequence for not doing so.

What a brand actually does is pick a fit model: one real person whose measurements define the base size. Every other size is generated from that base by grading — applying a set of increments to each measurement.

This means two things immediately. Sizes are defined relative to one specific body, and the shape of a brand's sizing is inherited from that body. A brand whose fit model has narrow shoulders produces narrow shoulders in every size.

Grading is why the extremes fit worse

Grading applies fixed increments. Go up one size and the chest gains 5cm, the shoulders 1.2cm, the sleeve 1cm, and so on.

Real bodies do not scale that way. People do not get proportionally longer arms as they get broader. So the further a size is from the base, the worse the assumptions fit — which is why the middle sizes of most brands fit best and the ends of the range fit noticeably worse.

It is also why extended ranges are a genuine investment rather than a formality: doing them properly requires re-cutting the pattern rather than extending the grade, and that costs real money.

The drift

Sizes have been getting larger for the same nominal number for about sixty years, steadily and in one direction.

The mechanism is unglamorous. A customer who fits into a smaller number feels better about the purchase and is marginally more likely to buy. Any individual brand that shaves a centimetre onto its size 10 gains slightly. No brand gains by moving the other way. Repeat across an industry for decades and the numbers detach from the measurements entirely.

This is usually called vanity sizing, which makes it sound like a deception aimed at the customer. It is more accurately a collective drift that nobody chose and nobody can unilaterally reverse.

The practical consequence: a size number is only comparable within one brand, and only within one season. Brands re-cut, change fit models, and quietly adjust their grade.

Why "the same size" varies between brands right now

Beyond drift, three things differ on purpose.

Target customer. A brand cutting for eighteen-year-olds and one cutting for forty-five-year-olds size the same number differently, and both are correct for their customer.

Country of origin. Sizing conventions genuinely differ. Japanese brands typically cut smaller and shorter in the body; American brands typically cut larger. An imported "large" is not a local "large".

Intended fit. A brand whose whole aesthetic is oversized builds that into the size, not only into the fit name. Their medium is somebody else's large, and they are not being inconsistent — you are reading the number as if it were absolute.

What to do about it

Stop treating the number as information about you. It is a brand's internal index. Its only useful property is consistency within that brand.

Keep your own measurements, and a set of measurements from a garment you like (see how to read a size chart). This converts every size question into arithmetic.

Learn your size per brand, not in general. Once known, it is stable for a few years, and it is the fastest shortcut there is.

Read reviews for the direction of error. "Runs small", repeated by twenty people, is more reliable than any chart, because it is measured against the population of that brand's customers rather than against a standard nobody follows.

Expect to be two or three different sizes, simultaneously, across the brands you own. That is the normal condition and not a sign of anything.

The short version

• No standards body defines a clothing size. Brands define their own from one fit model.
• Grading applies fixed increments, so the middle of a size range fits best.
• Sizes have drifted larger for decades because every individual brand gains slightly by drifting and none gains by reversing.
• The number is an index into one brand's system, not a fact about your body. Measurements travel; numbers do not.