Dress forms have been hand crafted for centuries and date back to the time of Egyptian pyramids. A wooden dress form was actually found in the tomb of King Tut and thought to display his clothing. You can read more in my journal "Toile and Stockman", about the custom dress forms created for clients at Yves Saint Laurent.
Fit has always been an issue in the industry mainly due to the fact that very few women are the exact same shape. I purchased my first dress form in college and it was originally made in the 50's or 60's. It reminds me of the quintessential "Barbie" body with a 36" bust line and size 2 hip. Pictures from that decade would show that was actually the ideal body and I'm sure the dress form represented that era. Body acceptance and positivity have made a "real" body the norm, not the exception and thankfully one company is leading the industry in both fit and realism.
Alvanon is the industry leader in anatomically developed dress forms. Developed in 2001, they use a device called AlvaScan to create their forms based off full body laser scans to capture the most accurate measurement data. It is then transferred to a computer and used to create a 3-D virtual body, which is the basis of their sizing. The empirical data is compiled from thousands of body shape scans.
They also create mannequins based on regions of the world, since the western body distributes weight differently from women in Asia. Dress forms can be purchased as general "standards" or custom forms can be purchased based on a companies own customer measurements. In 2012 Alvanon introduced Global Plus forms for sizes 16 - 24. You will still see differences in sizing among brands as companies cater to their own demographics. A size 8 at Target may not fit like a size 8 at Ann Taylor, but with most companies in the industry using Alvanon, clothing now has a higher standard for fit and becoming more inclusive.