I think the fascists' fixation on Sydney Sweeney has less to do with the size of her breasts and more to do with her blonde hair and blue eyes. They're thrilled to have an Aryan sex symbol in the mainstream again. They've felt threatened by the suggestion that other types of people can be considered attractive as well.
They also seem to be reveling in the fact that western culture is still superficial and celebrates people for being hot, that physical attractiveness is seen as virtue. Because fascism is an ideology predicated on superficiality, appealing to the most shallow of basic human instincts, they need surface-level aestheticism to peddle their rotten ideas.
Now, let me address the two big elephants in the room. I said I wasn't gonna talk about her boobs but they're not entirely irrelevant to the discussion. Skinny women with big tits represent the mainstream ideal of what straight men like, and fascists want to assert that what straight men like is what's right and true. The cis-hetero male-dominated beauty standard is the only one that should exist in their minds.
In reality, people (straight men included) like all sorts of bodies. But fascists don't like variety and diversity, they want a singular standard for society to adhere to. They want to reduce the complex and subjective experience of being human into something simple, a lie that's easy to understand.
you're right, it's not that the USA ever stopped being fascist but I think those time periods are when the general public was at its most reactionary and jingoistic and when these attitudes were reflected in mass media