The Anthropometry of Barbie

Barbie is a image of "the beauty myth" or ideal body type for ever women. But before Barbie was Norma which was muscular, weighted more, and over all had a bigger stronger looking body than Barbie. I would like to question what was it in women that thought looking skinner was beauty? I find the idea of beauty somewhat twisted and distorted from a feminist point of view when women have always competed with men to be there equals. As men started getting less fat and more muscular women just lost weight and became less muscular. I would like to know why?