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It seems as if our understanding of the concept of "thin" is being adjusted upward. I see plenty of teenagers and 20-somethings who look soft, round, and out of shape, but who probably consider themselves thin - and I guess they are, relatively speaking.
The 50's and 60's weren't long ago at all, but have you tried fitting into anything "vintage" recently? At a modern size 8, I can't find any of those stylish cocktail dresses from 40 or 50 years ago that will fit me. This tells me that we have gotten a bit larger between the end of WWII and today.
While I think that imposing rigid ideals is certainly dangerous, on the other hand, I think we may be "accepting" ourselves into a huge health crisis if the definition of "thin" keeps inching upward.