Why do men choose to date/sex/marry slimmer women over larger women?

Preface
Matt Ridley in his book, The Red Queen puts a very good case for why men prefer slim and thin women and not fat and huge one. 

Thinness as a factor of beauty is a recent invention.During the renaissance, it was quite opposite. Huge fat bulky women were consider more elite than thin skinny one. This was due to the obvious reasoning that the weight of the body defines the financial status of the women. The queen and the royal ladies were having good hygienic food to eat, less to work and hence were healthier and, as it turns out, more huge than, say, the wife of a peasants or farmers who had to work harder and have less hygienic food.

Portray of women during the renaissance period

An add during the early 20th century showing how to add weight if you are skinny.

Fashion = Beauty

Wallis Simpson has a quote saying  "A woman can never be too rich or too thin"

Fashion changes with time. And, we can know, beauty is subjected to fashion. So indirectly it also changes. Beauty of the women is a moving target. We need not rely only on our own culture for evidence that plump women can be more attractive than thin ones.

Robert Smuts of the University of Michigan has argued, thinness was once all too common and was a sign of relative poverty: Nowadays, poverty-induced thinness is confined to the Third World.

In the industrialized nations, wealthy women are able to afford a diet low in fat and spend their money on dieting and exercise. Thinness has become what fatness was: a sign of status.

Smuts argues that male preferences, keying in on whatever signs of status prevailed, simply switched. They did this presumably by a switch of association.

A young man growing up today is bombarded with correlations between thinness and wealth, from the fashion industry in particular. His unconscious mind begins to make the connection during his critical period, and when he is forming his idealized mental preference for a woman, he accordingly makes her slim.

Miss Americas falls steadily year after year. So does that of Playboy centerfolds. Both categories of women are 15 percent lighter than the average for their ages. Today thinness defines "healthier food with low fat and calories". That is a costly thing to purchase as compared to other food. 

PS- My answer is what I read in Matt's book. Most of the wordings are borrowed from the book. Emphasis are mine.