This trope is OlderThanDirt. While the Baroque paintings of Peter Paul Rubens gave rise to the term "Rubensesque", heavy-set women have been praised as sex symbols since prehistoric times as a sign of maternity. Many an academic has theorized on when and why slim women became the standard of beauty in much of the world, but the social and political aspects of women's body image issues are well beyond the scope of this wiki. What's important to know is that the love for big girls has never fully died down and is still dominant in many cultures.
Like men, women come in a full range of natural underlying builds. A different skeleton will 'wear' fat and muscle tissue differently so a weight that would probably be seen as unatractive on a more lightly built woman may 'work' on a naturally wider lass.
%%This means don't insert your favorite theory here.