The cultural standards by which a woman's weight and shape have determined her sexiness have changed dramatically over time in Western/European societies. For example large, curvy "rubenesque" women were deemed sexy for much of the 17th and 18th centuries whereas today, thinness is praised > curves.

Your premise is really a pretty bold claim and mostly seems like a simplistic misinterpretation of art, it has been addressed here many times, for example here https://www.reddit.com/r/AskHistorians/comments/4e8c52/did_heavier_men_and_women_used_to_be_considered/ by u/chocolatepot , who also, if briefly, addresses male beauty.

In any case, one should really differentiate between various "considerations" of "sexy/attractive":

Is a depiction meant to be idealistic or maybe just realistic, is it about some general expectations from the opposite sex, or rather self-image, or artistic ideas, or tastes of particular artists (what's their sex? sexual orientation? status?), or just detached symbolism, or idea(l)s of particular groups... All of those are related yet ultimately very different questions requiring their own kinds of sources.

Just consider how you'd evaluate what modern bodybuilding vs regular fashion magazines vs pornography vs various movie genres vs what men/women expect vs what they fantasize about vs what they tend to end up with (don't) say about beauty ideals...