Buying two garments in the same nominal size from different brands and finding them entirely different is not a quality failure. It reflects that sizing systems are proprietary and always have been.
There is no governing standard
National sizing standards exist in several countries and are voluntary.
Brands develop their own size charts, based on their own fit model and their own target customer.
Which means a size label communicates a position within one brand's range and nothing across brands.
The system persists because changing it would require coordination nobody has an incentive to initiate.
The fit model
Most brands develop patterns on a single person whose measurements represent their base size.
Every other size is derived from that base through grading.
Which means the whole range reflects one body's proportions, scaled, and it will fit people with similar proportions well and others poorly regardless of measurement.
Two brands with identical size charts can fit completely differently because their fit models had different proportions.
Grading
The process of scaling a pattern up and down, applying increments to each measurement.
The difficulty is that human bodies do not scale proportionally. Larger bodies differ in proportion, not just in scale.
Which means simple grading produces garments that fit progressively worse at the ends of the range.
Brands that grade in multiple blocks, or develop separate patterns for size groups, achieve better fit across a range and spend considerably more to do it.
This is a substantial part of why extended ranges are less common than demand would suggest — done properly, it is not simply making the same pattern bigger.
Vanity sizing
The documented drift of size labels downward over decades, so that a garment labelled with a given number today has larger measurements than the same label historically.
The mechanism is straightforward — customers respond positively to a smaller number, so there is competitive pressure in one direction only.
Which has made historical comparison meaningless and contributed to the range of measurements now covered by any single label.
Ease
The difference between body measurement and garment measurement, and the reason a size chart does not tell you how something will fit.
Wearing ease is the minimum required to move. Design ease is additional room chosen for the intended silhouette.
A fitted shirt and an oversized shirt built for the same body have entirely different garment measurements, and both are correct.
Which is why garment measurements are more useful than size charts, and why brands that publish flat measurements make shopping considerably easier.
Fabric changes the outcome
The same pattern in different fabrics fits differently.
A stiff woven holds the pattern's shape. A drapey fabric collapses around the body. A knit stretches to accommodate.
Which means brands adjust patterns for fabric, and the same style in two fabrics may need different sizes.
The online returns problem
Return rates in online clothing are extremely high, and the majority of returns are size related.
Which is enormously expensive for retailers and environmentally significant given transport and the fate of returned goods.
Responses include fit prediction based on purchase history, virtual sizing tools, and simply publishing better measurement data.
The last is the cheapest and most effective, and its adoption remains inconsistent.
What actually helps
Knowing your own measurements — chest, waist, hip, inseam, shoulder — and comparing to garment measurements where published.
Keeping a note of what size works in brands you buy from, since it will not transfer.
And measuring a garment you already own and like, then comparing, which is more reliable than any size chart because it accounts for ease and fabric.
That last technique is the one that has saved me most, and it takes a tape measure and two minutes.
The thing worth saying
A size that does not fit is information about a pattern, not about a body.
Given that the number is proprietary, arbitrary and drifting, attaching any personal significance to it is attaching significance to a brand's internal filing system.
The alteration option
The response to inconsistent sizing that most people never consider.
Buying to fit the largest measurement and having the rest taken in produces a better result than compromising across the board.
Taking in is straightforward. Letting out depends entirely on how much seam allowance exists, which brings the whole thing back to construction.
Costs are modest relative to the garment, and a good alterations shop will tell you honestly what is and is not possible before starting.
Made to measure and bespoke
Two terms frequently used interchangeably and meaning different things.
Made to measure adjusts an existing pattern to your measurements, within defined parameters.
Bespoke creates a pattern from scratch for your body, with fittings during construction.
The price difference is large and reflects genuinely different amounts of skilled work.
Made to measure has become far more accessible, and it addresses most of what sizing inconsistency causes, provided the measurements are taken properly by someone who knows how.