The same labelled size fits differently in every shop, and it fits differently now than it did a generation ago. There is no shared standard obliging otherwise.
Sizes are set by each brand
Clothing sizes are conventions rather than measurements, and each brand defines its own size chart based on the body it designs for.
National standards exist in some countries, but they are guidance rather than obligation, and brands are free to depart from them entirely.
The consequence is that a size label describes a brand's internal grading rather than any external dimension a shopper could verify.
Why sizes drift downwards
Sizes assigned to a given set of measurements have shifted over decades, so a garment labelled with a smaller number today covers larger dimensions than before.
The commercial motive is straightforward, since customers respond positively to fitting a smaller size and associate that with the brand rather than the label.
Once one brand does it the others follow, because appearing to run small against competitors costs sales regardless of whether the garment fits well.
The fit model behind the range
Every brand develops garments on a fit model of particular proportions, and the whole size range is graded up and down from that single body.
Grading applies proportional increments to each measurement, which works near the middle of the range and fits progressively less well towards the extremes.
Brands whose fit model matches a customer's proportions feel reliably good, and that consistency is what produces loyalty to a label regardless of price.
Why online returns are so high
Without a fitting room, a shopper orders the size that usually works and discovers on delivery that this brand grades differently.
Ordering multiple sizes with the intention of returning most is a rational response, and it is now common enough to shape how retailers plan logistics.
The cost of processing those returns is substantial, which is why some retailers now charge for them or restrict accounts that return heavily.
What actually helps
Measurement tables giving garment dimensions rather than body sizes are far more useful, because they describe the item rather than an assumed wearer.
Comparing those dimensions against a garment that already fits well removes the guesswork entirely, since the comparison is between two physical objects.
Reviews mentioning height, usual size and fit are the other reliable source, because they describe the same grading problem from the customer's side.