Watson's theory, which applies equally to men, is that in the past man would have spent his life constantly looking for food. The reward for the hard work would be eating and gaining weight. Hence, if you are thin you may be constantly looking for the next meal. Content people have weight on them. That is why we hire thin people because they are discontent and will work harder.
Heavier people are more mellow and less successful.
Thin people, on the other hand, are so driven by the need to find that elusive happiness that they become overachievers. So worldly success may well come more easily to the slender. Kate Moss is probably the most famous thin person in the world and she's looking particularly sad. It may not be a moral weakness but in fact a thin person trying to find happiness in artificial opiates. Watson's interest is purely scientific.