From personal experience I suspect that clothes shops employ their own measurement guides when it comes to sizing. A few years ago I bought three t-shirts in the same size from three different shops: Marks and Spencer, New Look and H&M. I marveled at how different they were and had real life actual facts to back up my suspicions. There is a British standard for sizing but (and I’ve lifted this straight from the trading standards website) “Unfortunately, there is no requirement for manufacturers or stores to use the British Standard resulting in a range of size indications for the same size of garment from different sellers.” I thought I’d illustrated this quite well with the comparison I did a few years ago but recently I wondered if the issue of size is actually style over substance. 
The loosest fitting top I bought clearly has a detailing around the collar that wouldn’t look right if the t-shirt was tight. Similarly, perhaps the tightest fitting t-shirt was supposed to be that tight as a point of style rather than sizing. So I thought I’d do the white t-shirt experiment again and second time around I wanted to get a T-shirt from The Gap. I wanted a ‘control’ t-shirt from a brand known for making what is a staple item of most people’s wardrobe. The thing is that in The Gap they label their t-shirts with the much broader size guide of S, M and L. This only added to the notion that when I buy something as ‘simple’ as a plain white t-shirt and it does have numbered sizing, it possibly has been cut to fit a certain way. So with The Gap out of the picture I tried my very best to find 3 t-shirts that were similar in design from the same three shops as before.
Before I’d even put it on, the Marks and Spencer’s T-shirt (the one that was the biggest fit last time) still looked pretty big compared to the other two but it did have elastic in, which neither of the others did so I reckon it probably was supposed to have a tight-ish fit.
The H&M top certainly felt tighter, even though looking at it on there doesn’t appear to be much difference in sizing between that and the one from New Look. Lying them flat one on top of the other though, you could see that the H&M top is narrower than the New Look one. So what? Well, you might employ the rationale that -oh well, this size 10 from H&M is meant to be very tight and is not the type of t-shirt that I want- I shall go to an other shop and see if they have loser ones. Or as I suspect is more common, rather than go to the effort of moving on to the next shop you take various sizes of the same t-shirt off the rail until you find one that is not so tight and think you are a size larger. NO NO NO you’re not. The clothes are made smaller! Honestly they are, I measured them against the British standards sizing chart (which you can get from this website if you’d like http://www.tradingstandards.gov.uk/towerhamlets/size.htm). The size around the bust on the H&M top is 78cm wide which is at the lower end of the bust sizing standard for a British size 8. The bottom of the t-shirt is 88cm which is in the bracket for sizing of a British size 10. The New Look top has a similar deal with t
he bust at 86cm- the upper end of the British size 8 bracket and the bottom of the t-shirt being 92cm puts it in the lowest end of the sizing bracket for a size 12. I’m not sure if any of this equals a size 10…
So let’s dispense with the niceties of saying ‘it’s just about the design’, as I have the more cynical view that shops using sizing as a tactic to appeal to certain markets and demographic of customer. Perhaps Marks and Spencer are aware of how sizing clothes with smaller sizing even though they are a larger fit makes a customer feel good about buying the clothes, believing perhaps that they have lost weight. Maybe H&M are targeting a younger market and simply assuming that their chosen demographic want tight fitting clothes; I worked in a clothes shop, I know how this goes from my time as a fitting room attendant. As customers we shouldn’t fall for this. We shouldn’t let it phase us. We should feel secure in the knowledge that clothes sizing isn’t standardised and therefore the label on the clothes is not a real indication of a garments measurements let alone ours. Of course that does make shopping a bit tricky. The reality is that actually the number on the label not only guides us but it also effects us, as it would if sizing was standardised too. If it was though, at least it would be easier to tell how things were meant to fit because who goes shopping with a tape measure and a copy of clothes measurements from the British Standards Institute?
When buying clothes, as impractical as it seems it probably is best to ignore the label. It is meaningless. Pick things that you think will fit based on what they look like on the hanger not on what the tag in the collar says because it doesn’t really refer to anything and therefore doesn’t matter. Size is just a number baby and it’s a fairly arbitrary one at that.

