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: 
Art does not exist in a vacuum, but is rather only one part of the historical record. Just as people comment on our modern standard of beauty today, so does early modern writers on theirs. Fortunately, Baroque art dates from a recent enough period that the historical record is extensive.

For example, a 17th century commentary on a Van Dyck portrait of a heavier women states:

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> William Sanderson in the treatise *Graphice*, published in 1658, noted that a beautiful woman was to have "a noble neck, round rising, full and fat . . . brawny arm of good flesh. Such a lady possesses a goodly plump fat."
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> Wind, Barry. *A Foul and Pestilent Congregation: Images of Freaks in Baroque Art.* Routledge, 2018.
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Beyond artistic contexts, we also find examples of beauty being explicitly attributed to body fat:

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> The use of fat is . . . It fills up the empty spaces between the Muscles, Vessels, and Skin, and consequently renders the Body smooth, white, soft, fair, and beautiful . . . Persons in a Consumption and decrepit old women are deformed for want of Fat.
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> Bartholin, Thomas, Caspar Bartholin, and Johannes Walaeus. *Bartholinus Anatomy: Made from the Precepts of His Father, and from the Observations of All Modern Anatomists.* John Streater.
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Historians, on the basis of information like this, therefore argue that people of the period - at least in some countries - considered it desirable for women to have a bit of plump. 

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As for "rationalisation" - of course, you can definitely make the case that they only considered chubbiness beautiful because of the association with wealth. However, does that necessarily invalidate their standard of beauty? I believe this delves into a philosophical realm over how beauty should be defined, which is beyond the scope of history.
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.