Typically, she will try on a Brand Y size 8, rather then a larger pair of Brand X's. Whereas designers can design for themselves, a brand or line dictates how sizing would work, like Banana Republic whose designs are for a slimmer figure and are cut more straight. A professor once said to her that women over forty wear clothes, too, and that real women need clothing that fits. And it should look good and make the wearer feel good about herself, too. In proportions, every measurement is affected, and goes through computer grading programs.
Size depends on the amount of grade but fits the same ratio measurements as the original design.
When buying clothes, keep an eye out for some are made with a measurement sheet, while others are made from a pattern in-house. Some companies have a spec sheet they send to a factory to make the pattern. Better fitting garments are based on patterns that are created in-house, and so the designers have more control over the fit. More care is taken with shapes and fabrics, and the designers use more subtlety than if the piece is banged out for a specific price. No wonder why designer brands attract such a following, and command such high prices for the clothes.