Maybe if the media didn't constantly shit all over women and how much they weigh, as if that had literally anything to do with their value as a human being, then she wouldn't feel the need to restrict herself like this.
But Victoria Beckham is an enthusiastic part of that whole media culture. She's long ago been rich enough to retire entirely from public life, but she actively courts publicity for 'brand Beckham', these days mostly through her children.
Also, she has set up her own fashion line which famously only caters for women as thin as her. So to me she is hardly some innocent victim of fashion and its very narrow idea of what women should look like. She's very much a part of it.