When a designer or a retailer are designing a collection they always have their own idealised customer in mind. When a Scandinavian designer is designing a garment they base it on their idealised customer while an Italian will do the same on theirs. As a result, their dress sizes will differentiate from another e.g in height and bust size. That means the fit of the garments will be very different from another.
Secondly dress design is based on proportion, meaning one dress design can’t fit and look good women across the sizes e.g. 6-20. There is a perfect dress for every woman but it is not the same one. You need to know yourself before you dress yourself.