According to this article at Scientific American, stressed out men prefer larger women.

The stressed out guys preferred a larger body size than their relaxed counterparts – but that was not all. “Men experiencing stress not only perceive a heavier female body size as maximally attractive, but also more positively perceive heavier female body sizes and have a wider range of body sizes considered physically attractive,” explain the authors.

The authors offer an evolutionary explanation. These sorts of explanations are always interesting, but whether they are legit or merely just-so-stories remains to be seen.