Anyone who collects or wears vintage clothes knows that commercial sizes have dramatically changed over the years. There was never a need to arbitrarily assign sizes to clothing until the ready-to-wear industry took off in the early 20th century. The earliest ready-to-wear garments were shoes, and their sizing system was developed during the early 19th century that was based on a percentage derived from an actual measurement of the foot. The earliest ready-to-wear women’s clothes were marked with a size number that was an actual measurement, usually the bust – ’36’ for example. Men’s dress shirts are still marked using actual measurements: 17/34 refers to a neck measuring 17 inches, and an arm length measured from the centre of the nape to the wrist of 34 inches. But for women’s clothes, actual measurements were displaced in the late 1920s by a size number that was arbitrarily used to stand in for actual measurements.
In an attempt to create a better standardized sizing system, the U.S. government commissioned a study of women’s sizes in 1940, in part to be able to create wartime uniforms for women in service. This study of 15,000 women took into consideration proportions as well as size. Ultimately, it was realized a standardized system could not work for all women as there were too many variants, which lead to several addenda including: half sizes, tall sizes, petite, misses, and junior sizes. In 1958 the American National Bureau of Standards created a publication to explain how the system worked – for example, a standard size ‘8’ consisted of a 31 inch bust, and 23 inch waist, while a size ’12’ had a 34 inch bust and 25 inch waist.
In 1970 the system was revised, the result of which was an increase of about an inch to all measurements. This was because of the new aesthetic for easier fitting clothes – it had nothing to do with a growing weight problem in the U.S. population. In 1983, as part of its deregulation spree, the U.S. government abandoned overseeing a standardized sizing system and left it up to manufacturers to create what worked best, which turned into a vanity sizing free for all.
Manufacturers quickly realized they could reduce the size number, flattering potential shoppers into thinking their clothes fitted their unrealistic size. Over the next 20 years size numbers gradually reduced. By 2011 a size 8 now boasted a 36 inch bust and 30 inch waist, and a size 12 had a 39 inch bust and 32 inch waist.
By 2000, the concept of a size 0 was introduced. This unrealistic pretension measured 26 at the waist and 32 at the bust, a size that is significantly larger than the 1958 size 8. Even a double zero could not match the 1958 size 8 at the waist. For more information check out this article and the links contained within.
Added October 31: I was following up on some fact checking with this, and found there is a LOT more to the story — there is a thesis in this ridiculous topic! Here is a link to another article with more detail.