What is Vanity Sizing?

According to Wikipedia, vanity sizing is "It is generally acknowledged that clothing of the same nominal size has become larger over the years. In 2003, a study that measured over 1,011 pairs of women's pants found that more expensive brands tended to be smaller than cheaper ones of the same nominal size. The increasing dimensions of garments of the same nominal size has caused some designers to introduce size 0, 00, or other subzero sizes."
However, it has been my experience that more expensive brands tend to be larger than cheaper brands.
Here is an example of vanity sizing that I experienced:
I went to a discount store to try some jeans on. Most of the jeans in the store cost $15-$25. The jeans that fit me comfortably here were a size 10. Then I went to a name brand store where the jeans cost $50 and up. Here I fit comfortably in a size 6.
Some speculate that vanity sizing is a way to lure customers to spend more money on their clothes, by appealing to their sense of vanity. Even though your weight and appearance are the same, the idea that you are a size 6 instead of a size 10 can be appealing.
Men's clothing don't tend to vary as much, probably due to the sizing structures, which are based on waist and in seam measurements.