If you are a woman and you wear clothing I'm sure you deal with the same frustrations which I do when it comes to clothing sizes. I know of no reliable way to figure out my size other than putting the item on. I'm amazed at the popularity of online shopping, even shops that I buy from on a regular basis require the try on confirmation. Last year I bought two pairs of Gap jeans online after purchasing the first pair from the retail location. I chose colours not available at retail and ordered all of them in the same size only to discover they weren't all the same length even though they were labeled as the same. Recently, I went to buy another pair in an updated style with more stretch and I had to go up two sizes to get a comparable fit. I could do the smaller sizes up but the stretch of the fabric created a muffin top which is not normally an issue for me.
Here's an article from Time magazine with more background on the history of sizing.
I did find out something interesting recently though about the so called arbitrary sizing numbers. A pattern maker told me the numbers came from the original grading rulers. A size 10 was drawn on a line that was marked as 10, a 12 was on the 12 line etc. There was a single number per size notation as grades are proportional. We do not make every measurement larger by the same amount, some areas change by an eighth of a inch while others by a quarter, etc. If you've ever looked at a nested sewing pattern you can see this across multiple sizes. We now rarely use those rulers so the origin of the sizing grades has been lost to modern pattern makers. If you look at the images below you can see the changes in sizes are not all done in the same increments.