To make matters worse, many companies use 'vanity sizing' to sell more clothes to people by appealing to their egos. They downsize the clothing labels. For example, they'll mark a size 12 to a size 8.
On the flip side, there are some Asian manufacturers who size their clothing based on the sizing of their local population, so their version of a "small" is really a child-size, whereas an XL might fit the average American woman, which according to Google (at the time of this post):
The average American woman is 5 feet 4 inches
and weighs about 140 pounds.
I can't begin to describe how frustrating it is to try on clothes at a store. I will grab a large range of different sizes to see which fits me better, grabbing everything from sizes 6-16. And I'm slightly below the average (according to Google).
For reference, I am fairly petite at 5'3, 120 lbs. I'm fairly fit -- I have a fairly active lifestyle and go to the gym on a regular basis. I also prefer my t-shirts semi-fitted or looser. I don't like extremely fitted t-shirts. Most clothing sales people assume that I wear a SMALL size or am about a size 6-8.
Ordering online is even trickier, especially if the store in question doesn't have an actual size chart to denote what they mean by small, medium, large, or extra large. I always have to click that little SIZE CHART link to make sure that I'm getting the correct size, which is not always a guarantee. At times, I have to read the comments and reviews (especially on Amazon) to see what consumers had to say. I've been saved from ordering the wrong size when customers complain about the sizing running too small or too big.
This discrepancy between sizes is seen clearly between all of the t-shirt companies that sell my beloved geek shirts. I have shirts from various companies and all of them FIT me exactly the same and all of them have different labels.
In the photos below, we have a t-shirt from TeePublic, TeeFury (which just recently changed their sizing*), and Her Universe. All three of these t-shirts are approximately the same size along the shoulder (given the differences between the collar sizes).
The widths of the shirts are within 3/4ths of an inch from each other and all fit me well.
Notice the sizing is XLarge - Large - Small, respectfully...of the same shirts as above.
All of these fit me very well, and yet none of them are labeled the same. This is why women sizing is Bantha fodder. This sizable problem has been occurring since sizing was introduced to clothing and women have been complaining about this issue for a long time. Mostly the clothing companies have ignored the complaints. (Vox gives a breakdown of the history of women's clothing sizes.)
Don't get me wrong. I love all of these t-shirt companies. But I would love it more if we adopted European clothing standards, which go by the actual measurements of a person's body, such as chest or waist or foot size, versus an arbitrary use of small/medium/large or US 0-24. It would really force manufacturers to a strict labeling standard and help consumers wade through the mess of clothing sizing. For example, I know that my shoe size is a EU 38. I can confidently purchase EU 38 shoes from any online shoe store and know that it will fit me.
I hope that one day, the US standardizes their clothing sizing and follows the European standards. It would help more consumers shop more effectively.
*For completeness within this article, TeeFury changed their women clothing sizing based on feedback from their female customers. I used to be a medium, but now am a Large in a Woman's sizing according to their charts.