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: 
A lot of the paintings were commissioned as portraits, why would people pay for themselves to be depicted in an ugly way? 

Wealth nowadays is associated with a slim, tanned, and shaped body because those are traits of people who have enough free time, and money to achieve it. In that period, it would be the reverse, being more on the fat side would require wealth, and more refined foods which were more expensive, while majority of commoners would be slim, tanned (for working in the sun) and toned due to hard work. 

It makes sense based on those, that it must have been acceptable to be fat back then, even desirable.
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)