I promised you a post with this subject, so I'll give it a go. I really don't think I can cover all the different aspects of this sticky topic in one post, but I may have a little bit of extra insight. But, more about that later.
First of all, don't we all hate size tags? And the fact that they always
seem to be wrong? Sewing your own clothes may seem like a way out of the limbo of the changing rooms, but be honest, is it really? Every pattern you buy comes in sizes and now, you have to try and pick the right one without being able to try stuff on...
These are just two examples of sizing tables. The pink one is Burda's, the grey one comes from Knipmode (where "bovenwijdte" = "bust measurement", "taillewijdte" = "waist measurement" and "heupwijdte" = "hip measurement").
You may have wondered about those ever-wrong sizes. Why are you a size 34 in one store and a 38 in the other? Why do you often have to pick a larger size than your usual RTW one when studying the sizing tables of sewing patterns? Isn't there some kind of standard?
Well... That's the point, isn't it? There is no standard. There is no rule telling fashion brands or pattern makers that such-and-such are the measurements for each size.
The only regulated sizing I know of (and I haven't made a study of this, so there may be many more cases) was in the US in the 1950's when sizing for womenswear was done by bust size, in three different shape categories. In fact, more than the confusing sizing, I think it's a pity we lost the notion of different clothes intended for differently shaped women in today's fashion jungle. Although it seems we just have many more different shapes nowadays...
As a 'rule', fashion companies (for RTW and sewing patterns) try to size their products according to their customer's expectations. That's the reason sizes run (much) smaller at youth-oriented stores like H&M. Not just the number stuck on the main size (bust circumference for dresses and tops, hip circumference for bottoms) differs, but also the ratio between measurements. Again, this is based on the target customer.
A company which only sells in the Netherlands, or northern Europe (like sewing magazine Knipmode) will make its designs for rather tall women (Dutch women are tall, on average). And a company selling mainly to 35+ women will decide on a much curvier basic block than one selling to ages 15 to 25.
In RTW, you would do well to seek out and remember the brands which best fit your body shape.
You probably knew most of that already. However, there's another nastly little secret which will sound familiar to the experienced seamstress.
Fashion companies have to start somewhere when making a new design. In theory, this starting point is an ideally shaped specimen of the body type of their target audience, in their most average size. However, many brands have to present clothes to retailers to sell them and for that reason, they may prefer to go with a smaller than average size which looks better on a hanger.
In the Netherlands, the size most brands do their fitting for is 38. Their own 38. Usually combined with a height of 1,70 meter and a B cup. That's why I said seamstresses would recognise this. This is what all the talk of full bust and small bust adjustments is all about. It's why they're needed. And why anyone with a seriously different cup size has such trouble buying clothes ready-to-wear.
Another major cause of confusion is what is known as 'size inflation'. Apperently women are more eager to buy if they are pleasantly surprised by the size on the tag. To use this effect even while people are generally getting larger, companies regurlarly adept their sizing tables. Those selling sewing patterns do this rarely. They know their customers are used to go by the sizing table and will only change it if they feel there is a real, over-all change they have to respond to. For example, about two years ago, Knipmode changed its sizing. They made each size slightly bigger but, more importantly, they changed the standard height from 1,68 to 1,72. To suit the afore mentioned tall Dutch woman. In doing so, they lost me as customer because my measurements had now pretty much dropped off the chart.
Ready-to-wear manufactures live by their ability to respond quickly to the market they're in. They employ people who constantly monitor the application of their blocks and adept the size and fit of those blocks to customer demand. So they may change quickly and often.