A size fourteen from one retailer will not reliably fit somebody who wears a size fourteen from another. This is universally experienced, widely complained about, and structurally unlikely to change.
Why there's no standard
Sizing standards exist — various national and international standards bodies have published them — and adherence is voluntary in most jurisdictions.
There's no legal requirement for a garment labelled a particular size to conform to any specification. The label is a manufacturer's description rather than a regulated measurement.
Which means each retailer develops its own size chart, based on its own fit model and its own target customer.
Why the numbers drifted
The phenomenon frequently called vanity sizing — the observation that garments labelled with a given size have become larger over time.
Comparisons of garments from different decades against their stated sizes show substantial drift, with a modern garment of a given size measuring considerably larger than the same nominal size decades earlier.
The commercial logic is straightforward. Fitting into a smaller number is experienced positively, which creates positive association with the retailer, which affects purchasing.
Once one retailer does it, others follow, and the numbers detach progressively from any fixed measurement.
The consequence is that size numbers now convey almost no information about actual dimensions, which is a strange outcome for a labelling system.
The body shape problem
A more fundamental issue than the numbering.
Garments are designed around a fit model — one person whose measurements the pattern is developed against — and graded up and down using ratios.
Grading assumes bodies scale proportionally, which they don't. Someone at the upper end of a size range doesn't have the same proportions as the fit model scaled up.
Which is why fit deteriorates at the extremes of a retailer's range, and why the same size can fit well in one garment and badly in another from the same retailer.
Anthropometric surveys have documented the range of body shapes within any given set of measurements, and the variation is substantial enough that no single grading system fits everyone.
Why online retail made it worse and better
Worse: buying without trying means relying on a size label that conveys nothing. Return rates in online clothing are very high, and fit is consistently the leading reason.
Better: the same pressure has produced improvements. Retailers with high return costs have incentive to improve sizing information, and some publish detailed garment measurements, model measurements and fit feedback from customers.
Customer fit reviews — whether an item runs large or small — turn out to be among the most useful available information, and considerably more useful than the size chart.
What actually helps
Know your measurements. Bust, waist, hips, inside leg, and for tops, shoulder to shoulder. These don't change with retailer.
Measuring takes five minutes and makes every size chart usable.
Use garment measurements rather than body measurements where available. Some retailers publish the actual dimensions of the item, which is far more useful. Comparing against a garment you own that fits well is the most reliable method available.
Read fit reviews. Particularly from people who state their own measurements.
Check the fabric. Stretch content changes how much fit precision matters. A woven garment with no stretch requires accuracy; a jersey doesn't.
Consider alteration. A garment that fits at the largest measurement and is altered elsewhere frequently produces a better result than any off-the-peg compromise. Alteration is cheaper than most people assume and transforms fit.
The psychological dimension
Worth naming because it's the reason this generates distress rather than mere inconvenience.
Size labels have acquired meaning beyond measurement. Research on body image has examined how sizing experiences affect mood and self-perception, and the effects are real.
Which produces a situation where an arbitrary number, set by a retailer with commercial motives, affects how somebody feels about their body.
The useful reframe is that the number describes a garment, not a person. A garment that doesn't fit is a garment that was designed for different proportions, which is information about pattern grading rather than about you.
Some retailers have removed numerical sizing in favour of measurements, and some have removed sizes from labels entirely so that they can be cut out. Both address the symptom rather than the cause and both seem reasonable.
Whether it will change
Unlikely to standardise voluntarily, since differentiation serves retailers and standardisation would remove a marketing tool.
What may change it is technology. Body scanning, virtual fitting and made-to-measure production at scale would make sizing categories less relevant, and several attempts at this exist with mixed success.
In the meantime, measurements remain the only thing that means anything, and treating the label as a rough guide rather than a fact is the practical accommodation.
Buying for children
Where the problem is compounded, because children's sizing is nominally by age and children of the same age vary enormously.
An age label is close to meaningless as a measurement. Height-based sizing, used by some retailers, is considerably more useful, and measuring a child's height and using that against a chart produces better results than guessing from age.
The practical implication for anybody buying gifts: ask for height rather than age, or buy larger. Children grow into things and cannot grow back into them.