One of the biggest changes I've noticed in my own lifetime is the shift away from more slender females and toward more fleshy ones. We see this at the level of the average girl, celebrity sex symbols, porno chicks, phrases that guys use to describe what they like, and so on.
Now I don't mean BMI, waist-to-hip ratio, etc. No existing measurement captures the intuitive sense of whether a girl has a small or large amount of fat primarily on her boobs, ass, and thighs. BMI only tells you how fat or skinny she is, ignoring body shape. Waist-to-hip ratio only tells you how hourglass-shaped she is, ignoring how much meat she's got on her bones. I'm talking what distinguishes Marilyn Monroe from Paulina Porizkova, both of whom are a healthy BMI and with hourglass figures, but where one is clearly more buxom and bootylicious than the other.
The first in-your-face signal for me was in 1992, when Sir Mix-a-Lot released "Baby Got Back". The video was in frequent enough rotation on MTV, it hit #1 on the Billboard charts, and even us fifth-graders were buying the single on tape. Since then the popularity of big ol' booties and fake tits has only skyrocketed. Later in the '90s there was J.Lo, then Beyonce in the mid-2000s, and now... I don't know, who's the reigning booty queen these days? Lady Gaga or Shakira, I guess. Or Kim Kardashian?
I'm aware that certain female celebrities have been going in a more 12 year-old boy direction, like fashion models and butt-kicking babe characters in movies. But when it comes to who the average guy is thinking of or looking at while he jerks off, it's more likely Nelly Furtado than Uma Thurman.
Was there another time when fleshy girls were in high demand? Yep: the heyday of pin-up girls from the later 1930s, '40s, and '50s. What does that period have in common with the present age of curvy-mindedness, which began around 1992 with "Baby Got Back"? Falling violence rates.
Is the other correlation there, between rising violence rates and desire for slender girls? Looks like it. As crime rose during the '60s, the fleshy girls began to give way to the more waifish ones like Audrey Hepburn and Jean Shrimpton. They were basically gone by the '70s, when Farah Fawcett and the chicks from ABBA got the most attention. I can't think of a single '80s sex symbol who was fleshy... unless you count Jessica Rabbit, but she was a conscious throwback to the '40s. Even during the first couple years of the '90s, the parade of babes on Twin Peaks were all slender.
Going back to the previous rising-crime period, ca. 1900 to 1933, I'm not sure about the first half of that period, but my impression is that they were slender too. During the Jazz Age, though, the demand was definitely for slender girls, including the original "It Girl" Clara Bow. Lithe flappers who taped down their breasts probably were an extreme form, but still their popularity shows that what men wanted was more in the slender direction. That lasted at least through 1933, when Fay Wray represented beauty in King Kong.
While acknowledging the variation we see in any slice of time, the swing of the fleshy-or-slender pendulum over time does appear to track the cycle of violence rates. Most of this evidence is from male preferences, but there could be a female supply-side effect too -- maybe women's bodies respond to their perception of whether the world is getting safer or more dangerous. That would require a large representative sample of women throughout the past 100 years, though, so I'm restricting things to the demand-side of male preferences.
My interpretation of this pattern is that a fleshier woman is seen as an investment into the far and stable future. Those fat reserves are a kind of energetic savings account, and why would you bother saving unless you believed the far future would be reachable? Men planning their family formation on the assumption of a safer future will want a woman with extra flesh just in case something goes wrong now and then.
But when the world is getting more dangerous, you don't care what extra value she might have into the far future -- you just want her to be able to pump out some kids now and hope they make it, perhaps on their own. You also might count it against her if her body shape suits her to hanging around the house mothering her children, when she may have to be on-the-go -- not in the sense of being a careerist or breadwinner, but simply not being able to stand still while so much commotion is going on in her environment.
If they don't look so maternal, the rising-crime-era chicks do look much more adorable. You see it in still pictures, movies, and books. Fitzgerald is always going on about how expressive, soulful, and hypnotic the girls' eyes were in the Jazz Age, and it was no different from the '60s through the '80s, from Audrey Hepburn to Kelly Kapowski.
It's not primarily a neotenous thing. It's more the look of an abandoned puppy who's trying to win over a prospective owner with its cuteness. The pin-up dolls didn't look like that, and neither have girls of the past 20 years. The connection to rising vs. falling violence rates is straightforward: when times are more dangerous, you need to rely on others more, especially male protectors, so girls have to put on a sweeter face. When they're getting safer, girls don't need to recruit as many male protectors into their social circle, and so don't have to crank up the wattage in their eyes.