The sizes are exactly the same in 2016 as they were in 1920 or before. It's standard measurements of the width of the lower part of the needle. The needle shape has varied more over the years; the scarf was introduced in the 1950s I think, various needle tips have been developed, and the thread hole can vary, the grooves on either side of the shaft, etc.
I think the differences in recommendations have more to do with the fact that more than a single needle size will work just as well for the same fabric and thread. There are all kinds of factors to adjust for; different fibers and blends in the fabric, differen weaves, thread sizes.
Before the 1950s thread was made of cotton, silk or linen. Silk and linen was strong but more expensive, and cotton needs to be quite a bit thicker for the same streangth as todays sew-it-all polyester; and thicker thread needs larger needle hole, which means larger needle. Fabric types thought of as medium weight is within a range of thicknesses and weave types, and isn't close to any fixed standard like metric or imperial measurements. I guess it has a bit to do with preferance too. Just look at the blogs on quilting, people use and recommend different needle size for the same jobs.
Last edited by Mickey2; 03-06-2016 at 04:58 AM.