Question: I've often heard that the obesity of women in baroque art signs that such women were the ideal of beauty. How could we know that? And how could we know it wasn't just "cultural rationalization" of a standard physical appearance of higher classes?

Answer: 
This aesthetic relativism is also confused by the fact that for most of the history of Western art, women weren't able to model so the artist often just changed male bodies (from his/her model) into female bodies by adding breasts or extra fat or whatever.

Here's a source: <https://renresearch.wordpress.com/2011/02/11/men-with-breasts-or-why-are-michelangelos-women-so-muscular-part-1/>

Which references Gill Saunders' *The Nude: A New Perspective* (1989)
The idea that, taken across the entire range of media and works, art from any given period represents a society or culture's ideal for how anyone - male or female - should appear has no real basis in fact.

It is vastly more likely - especially in any time frame where we know the names and personalities of the artists being discussed - that any given work represents an example of what *the artist* considered *technically* interesting or challenging, or important in some other way by the standards of his craft or in the judgment of his contemporary artists. This was true even in portraiture.

Three centuries of artistic technical fascination with chiaroscuro *alone* should show us this. Artists chose bodies, poses and themes which allowed them to explore the interaction of light and shadow and the human body; and they did so for technical (and aesthetic) reasons that only rarely had anything to do with depicting ideals of physique.

For most of the history of representational art across all cultures, in fact, there was little attempt to make the figures depicted realistic in any way - "beautiful" or not. Nobody would argue that Greek sculptures from the Geometric or Archaic periods represent the cultural ideal of male and female physical beauty of the time, or that medieval weavers of tapestries or illuminators of manuscripts were attempting to depict the perfect human form.