It would appear to me, based purely on my own musings (grin) that the definition of 'beauty' as it applies to body weight tracks closely with 'affluence.' In the Middle Ages1 (from whence we have many, many paintings, woodcuts, etc. of women considered 'beautiful' by their contemporaries) most women, when found desirable, exhibit Rubenesque characteristics. Heavy was beautiful, not only for those wonderful curves, but also because being heavy in a time of subsistence agriculture meant that you commanded enough resources to be above your body's minimum weight.
In contrast, men's appearance was (and is still, at least in the latter case) closely tied to warmaking or providing. Both of those, in those times, involved being in really, really 'good' shape - try wearing plate mail for a few minutes, or farming ten acres of marginal English countryside, rocks and all, with only hand tools and ploughs (draft animals were for the rich) and see what it takes.
In any event, the situation shifts as the diet of the 'lower classes' shifts. It's actually really, really hard to get fat on grain and vegetables, which was much of the staple at those times. As we enter the Age of the Machine, however, the 'workers' move to the cities, and a change in diet begins - cheap food is now food that's easy to transport and store, which means canned or prepackaged. Since canned or prepped food (especially in the early days) really didn't taste anything like 'the real thing,' the sure-fire way to both get people to eat it as well as save money on supplies (once trade with the Caribbean2 wasn't a problem) was to load in sugar. Even today, look at the ingredients of canned 'prepared' food - fat and sugar prevail (Spam, Corned Beef Hash, even Candied Yams in a can, for example). Ketchup? Lotsa sugar. McDonald's Fries? Sugar added, at least until recently. The fat was a natural consequence of the cheaper provender finding its way into the cans, as well as its ability to 'satiate' the consumer quickly and hence for less lucre.
So, moving on. Slowly, as work became less physical and more stationary, the average phenotype began to gain weight. As things continued, people tended to need to remain indoors and busy to stay employed and paid; therefore, 'quick' foods and cheap foods, such as canned goods, candy, snack foods, etc. all became popular. With the rise in disposable income, the choice between convenience and healthy eating was viable, and many made the former.
Pin-up girls show the trend. Clara Bow (thanks to Akasha for a correction, here), the Original 'It girl,' weren't no scrawny chicken. She had thighs, Mort. Big ones. Breasts sturdy enough to hold you up, and nice enough to make you want to rest your head there. Lovely ones. Delic...er, sorry. Anyhow. Look at the calendar girls of World War II - if they're not Rosie the Riveter buff, they're definitely solid women, with real bodies. Marilyn Monroe was the tail end of this trend, as the pin-ups slowly began to become thinner. Why?
Because, as the earlier writeups in this node point out, weight management in a world of high-cal convenience takes a great deal of willpower or a great deal of time/energy, usually both. The average worker can't possibly undergo the kinds of swings that Drew and Kate are described as having above; nor can she (if slender) afford (or usually, thank God, desire) the 'enhancements' designed to transplant curves onto those skinny frames. As a result, the 'average' phenotype is now a bigger person, and if genetically slender (as opposed to situationally thin, which most people used to be) then not (usually) as curvily fun to look at.
Those that can afford to be slender, in good shape, and curvy in the right places tend to be those for whom doing such is...well, a job. Those who can do it professionally aren't that many in number; many who could do it choose not to because of the demands it can place on body and soul. Thus, the 'poster girl,' who is now just as unattainable to the average Joe as the heavy Queen or Lady or even poster girl back in Middle Earth, is now typically either a slim, yet curvy product of rigorous diet, exercise and surgery, or a completely waif-like skinny trading on their resemblance to 14-year olds (and mens' attraction to forbidden youth).
So. What are those who prefer to look at women with physiques, like, say, Lucy Liu or Rene Russo, or the (cherubically heavy version of) Drew Barrymore or Sandra Bullock, or (better) Rosie O'Donnell in Exit to Eden to do? We must find a way to make it hard to be heavy! If even the most impoverished countries of the world are beginning to see problems with obesity (not due to overnutrition but the wrong nutrition) as a report aired on CNN and company recently concluded, then those that look like they've escaped this situation are those that command attention from most.
Whew. Sorry. I'd even bet this is covered elsewhere in the database; it's just a meandering I often find myself travelling.
1. Well, damn it all..liveforever informs me that while my premise of thick-is-beautiful may have been true during the Renaissance (Ha! You'll notice my excellent da Vinci references later, smart me!) it wasn't true in the Middle Ages. Hm. As I already admitted I'm talking out of my fundament here, maybe the logic holds and the timeframe is wrong...it's true that most Medieval peasants didn't starve, being farmers after all. Earlier, in the Dark Ages, now, that would've been a time of skinniness, no?
2. Thanks to yerricde for the proper spelling of 'Caribbean.'