I have spent the last 2 days clearing out DM's attic, and found loads of old clothes up there (the house has been in the family a long time and has a large attic) including my great aunts wedding dress. She got married in 1960.
My dgm often referred to this aunt as being quite stout, and her wedding dress was a 16. I tried it on, as you do. I am a size 12, bmi of 25, 5'6". It was too small. By quite a margin. :( I am nearly 40, and have been aware that vanity sizing was on the generous side, but this has really shocked me.
Having spent the rest of the evening "investigating" this by trying on more motheaten vintage clothing we discovered a size 10 in 1960 was about a size 6 now. (Dsis is a 10 and needed at least 3 more inches around the waist, although the wedding dress fitted beautifully) and that it wasn't just the wedding dress that didn't fit me, there were 3 more dresses in there, all size 16, and all too small. :(