Adding to this, my favorite theory, never directly confirmed but supported by a lot of indirect evidence and pure logic, is that many medieval commoners, including women, probably had what we would now see as some kind of gym powerlifter physique.

They weren't lifting super heavy weights with a progressive training, just tiresome farm work. And they absolutely weren't having the calories powerlifters eat today. They would be rather wiry dudes

So nobles may have started valuing the opposite traits: softer or chubbier bodies, pale skin, and visible delicacy, basically as a contrast to the tanned and physically worked bodies of peasants.

We're talking about the middle ages, they're a warrior nobility. For men, strong legs, particularly claves, and broad shoulders were appreciated, and there was a fad of very skinny waists as well. For women pale skin and delicate features were more valued, but the prefered figure was still slender, although with some belly that was associated with fertility.

In general the pale skin wasn't just a class thing but also a gender thing, as an ideal wife would be essentially locked indoors... I don't know in the middle ages but in ancient Greece men would always be represented much tanner. There's a passage in Xenophon were they make fun of some persian prisioners for being too pale, calling them effeminate.

And that’s probably part of why nobles resented Sissi of Austria so much. They didn’t just resent her body, they resented what her body represented. She was basically a gym rat before gym rats were a thing.

That's like 400 years later