Beauty / Most runway models qualify as anorexic

Note: Most links here are NSFW.

Friend of mine posted this link of Facebook (thanks Chris):

http://www.pajiba.com/trade_news/sidebyside-comparison-of-averagesized-woman-with-a-supermodel-will-blow-out-your-mindhole.php

And it got me thinking, in the Middle Ages / Renaissance model thin women were not considered the most beautiful. See Rembrandt's most famous nude painting:

http://en.wikipedia.org/wiki/Bathsheba_at_Her_Bath

But that was probably because the majority of people during that era were malnourished peasants who were dangerously thin. Now we have a society with the opposite problem where food is plentiful and the majority of people are probably over-nourished and the super-thin runway models are considered most beautiful. Does that mean beauty just defined as what most people are not? Like the trend in Japan now for women to have crooked teeth.

http://www.nytimes.com/2011/10/23/fashion/in-japan-a-trend-to-make-straight-teeth-crooked-noticed.html

Throughout history certain features are almost universally considered beautiful - i.e. certain face shapes / symmetries. But some aspects of beauty do seem to change over time. So I wonder how much of our conception of beauty is driven by society and how much by human nature?