Not to quibble, but I don't think that's a cornerstone of FEMINIST theology. I just think that's libertarian common sense. Don't men have a right to whatever kind of body size THEY want? Or does some bureaucrat somewhere now get to tell me to drop 20 pounds and firm up those sagging abs?
Of course people have the right to any body size they want. But feminists are often heard to complain about how society - read 'men' - enforce unrealistic body ideals on women.
Yes, "societal restraints" was an unfortunate phrase. What those feminists really want is for their physical appearances to play no role whatsoever in their attractiveness to men...and that is a futile attempt to repeal a law of biology.