The same nominal size differs substantially between brands, and the reasons are historical, commercial and largely unregulated.
The absence of a binding standard
Voluntary standards exist and brands are generally free to size as they choose.
Which means a number on a label is a brand's own designation rather than a measurement.
Studies measuring garments of nominally identical size across brands find substantial variation.
Fit blocks
Each brand develops patterns around a target body shape.
Which differs by brand and by market.
This is why a brand fits you consistently well or consistently badly across its range.
Size inflation
Garments labelled with a given size have grown in measurement over decades.
Which is documented in comparisons of historical and current garments.
Commercial motivations relating to how customers feel about the label are the generally offered explanation.
Regional differences
Sizing systems differ between markets, and conversion charts are approximate.
Which causes problems when buying internationally.
Actual measurements in centimetres are the only reliable basis for cross-market purchases.
Returns
Fit is the dominant reason for clothing returns, accounting for the majority in most reported figures.
Which is a substantial cost to retailers and a substantial environmental impact.
Better size information reduces it measurably, which is why retailers invest in it.
Fit technology
Size recommendation tools using body measurements or past purchases.
Which have reduced return rates where implemented well.
Accuracy varies and depends on the quality of the underlying garment data.
Measuring yourself
Chest, waist, hip and inside leg, compared against a brand's published size chart.
Which is the reliable approach and takes five minutes once.
Measuring a garment you already own and comparing is frequently even more accurate.
Vanity considerations
The number on the label has no meaning outside the brand that printed it.
Which is worth internalising, since it makes buying the size that fits considerably easier.
Shoes
Sizing varies by brand and by last shape as well as by regional system.
Which is why foot length alone is insufficient.
Width fittings are available from some brands and are frequently the actual problem.
Online fit information
Retailers publishing garment measurements rather than only size labels.
Which materially reduces returns and is still not universal.
Customer reviews commenting on fit are frequently the most useful information on a product page.
Return costs
Free returns shift cost to the retailer and are increasingly being charged for.
Which changes the calculation of ordering multiple sizes.
Statutory return rights for distance purchases remain regardless of policy on postage costs.
Inclusive sizing
Range availability varies enormously between brands.
Which is a commercial decision and has attracted sustained criticism.
Extended ranges are frequently sold online only, which limits the ability to try items on.
Alterations
A well-fitted garment altered from a larger size frequently beats an off-the-peg compromise.
Which is an underused option given typical alteration costs.
Fabric behaviour
Stretch, shrinkage and drape affect fit as much as measurements do.
Which is why composition information on a product page matters.
Natural fibres frequently shrink on first wash, and care labels state expected behaviour.
Body shape versus size
Two people with identical measurements in one dimension can need different sizes.
Which is why brands with different fit blocks suit different people.
Finding brands whose block matches your shape is more useful than knowing your size.
Menswear conventions
Measured sizing in some categories provides more information than arbitrary numbering.
Which does not eliminate variation, since manufacturers measure differently.
Children's clothing
Sized by age, which corresponds poorly to individual children.
Which makes height-based sizing considerably more useful where offered.
The practical approach
Know your measurements, read the brand's chart, and check reviews for fit comments before ordering.
Building a reference
Note which sizes fit you in the brands you buy, and keep it on your phone.
Which removes most ordering uncertainty for repeat purchases.
Measuring a garment that fits well gives you a target to compare product measurements against.
Why it will not be fixed
Standardisation would remove brands' ability to fit their target customer distinctively.
Which means variation is a commercial feature rather than an oversight.
Measurements published alongside sizes are the realistic improvement, and more retailers are providing them.
Sustainability angle
Returns driven by fit carry transport emissions and frequently end in disposal.
Which makes accurate sizing information an environmental measure as well as a commercial one.
Retailers publishing detailed measurements report measurably lower return rates.
A closing thought
The number on the label is a brand's internal designation and nothing more. Once that lands properly, buying the size that fits stops being an emotional decision and becomes a practical one.
Keeping your measurements on your phone removes most of the remaining friction from ordering online.
A note listing which size fits in each brand you buy is a two-minute document that saves an enormous amount of returned parcels.