Actually part of the reason we are getting thicker around the middle is due to the corset - or the lack thereof. (Now don't get me started on Spanx - those are a miracle garment if you are going to be wearing something clingy.)

I am also a fanatical sewer and I think the sewing sizes you see on patterns have been the same since the 70's. In ready-to-wear I'm now in a 6 or 8 (bottoms) but in a pattern I need to buy a 10 or 12.

And the term for the gradual enlarging of sizes is actually called "vanity sizing."