To sew old things

I bought this in USA. There probably is no woman who wouldn't look good in this type of blouse!
I'm only wondering. It says size medium on the package. Weren't all people like two sizes smaller 50 years ago? 
I remember when I used to try vintage dresses or skirts from 50's. The waist seemed to be fitted for a 10 year old, not a grown up. And it couldn't have been that most of the dresses in the shop were worn by teenagers! Besides, all the other parts indicated it used to be an adult who wore the garment.

I hope my fellow bloggers will help me with this. There are a couple of ladies who dress almost exclusively in vintage and know a thing or two about the old time sizes :-)