Wedding gown sizes are based on an older scale of sizes that have never been modernized. It's just a number, doesn't mean you are any bigger or smaller than what you are. A lot of stores have "vanity sizing" where a 6 is really an 8 but the smaller number makes women feel better. Wedding dress makers just have not changed from back in the day when a woman like Marilyn Monroe wore a 12.
"Bridal sizing goes back to a scale established during World War II that used data intended for making uniforms, Moore [senior vice president of merchandising and product development at retailer David’s Bridal] explains. The scale also was used for ready-to-wear clothes, but over time, sportswear adapted its sizes to reflect changing body shapes, while bridal, for the most part, didn’t."