We've actually discussed this here before, and it's a universal frustration! I believe Angie made the point that there are so many variables in fitting a woman's body that it's nearly impossible to standardize sizes, even using bust-waist-hip measurements, because things like rise, height, shoulder width, booty size, etc., go into determining a garment's fit. Then you have European sizes, UK sizes, Asian sizes, etc., which are all based on different fit models.
Each designer has the freedom to use a different type of fit model, and that can actually be a good thing -- one designer tends to cater to a more athletic body type for example, while another may use fit models with more curves, so different body types are, in theory, served by different designers. I can see this at play when I try on certain designers' garments time and time again -- some brands fit me great with no tailoring straight off the rack, and others never seem to work for me -- they're too straight, don't accommodate my chest or hips, etc.
You would think the designers could get it a *little* more consistent, though -- there's something a little ridiculous going on when I have everything from a 4 to a 10 in my closet and they all fit. In stores where I end up fitting into a very small size, like a 4, I can't help but think that the truly small woman is sized out of the equation. I'm 5'7" and 135-40 lbs, for crying out, I'm not really "small." And conversely, when I find myself having to size up to, say, a 10, and I notice there is nothing beyond a 10 on the rack, I get peeved on behalf of those larger than me who would like that item as well. So I think my campaign would be more for extended sizing.
