Well, if we're speculating, I'd start by turning the causal arrow around.

Body ideals and patterns of desire are certainly tied in to larger structures of meaning that become more or less salient to different cohorts and groups over time. And so you're quite likely to find different bodies admirable or desirable than your parents do.

As a regular reader of the Globe, I don't find anything written by Christopher Muther meant to be serious, scientific, or backed up by research.

I think there could be something in there, if only as a tangent. Most ideals or innovations seem to follow a generational pattern, so why not body image? it doesn't necesarily have to be our parents, but most young people go out into the world wanting to make an impression, and the body image is part of the landscape that we learn to take for granted. Thus, it's a possible target for change. Not a very educated analysis, but still.

Hmm... I watched without sound on and thus without commentary. What I saw was a bunch of thin women of varying breast size. As for the men, they were all lightly muscled. Not a whole lot of variation in "body types".

I agree that rebellion seems like a poor explanation for the change. I mean, we like what we like and after teenagerhood people who say "oh, it's just a rebellion" are actually just saying "I don't like the same things you do so I can't understand it so I'm going to pretend you're being immature."

However, what might make sense-- and this is of course completely not researched and just a hypothesis-- is a combination of changing media, and that we don't want to have sex with someone with a similar body type to our parents.

There's a lot written on the history of the body ... and there are all sorts of cultural, societal, and class forces to be taken into account in any era. The 1920s, to use but one of his examples, was not just populated by Flappers! Weight gain products for woman (and men) were common, because for some areas/classes being thin wasn't attractive as it was a sign of poverty.

However, for those at the top of the U.S. social pyramid larger bodies were easier to obtain, often their elders were 'rounder', and there was a large degree of generational rebellion in the Flapper body type. It was shocking to older people of a certain class, and it was hard to achieve and maintain.

A better (but still flawed) general rule when it comes to fashionable body types is what is the opposite of the easily achieved body in your society ... In lean times being rounder is usually desired. In times of plenty then being skinny is desired.

If being skinny becomes too 'easy' to achieve then you add in qualifiers, like being toned, having a large or small bust, being an hourglass shape, or being very flat. You can add in other factors as well, such as the colour and length of hair/nails/eye(lashe)s, and even 'once impossible to change' components like height, nose shape, etc.

My boyfriend reacted with indignation to the idea that there is a soul on this planet who denies Tom Selleck is "the cat's pyjamas".

I think it's just a matter of- we start to want something different. So one body type becomes popular and then we start to seek out something A-typical. It's not necessarily rebellion so much as the desire for change and novelty.

I was struck most by one of the first clips he showed, of the man and woman in the 20s wearing swim suits. The narrator referred to them as healthy young people. Perhaps the fluctuation in body size has to do in part with what American culture viewed as healthy, which HAS changed enormously from decade to decade, as well as which body types were viewed as the most suitable for gender roles. For example, the "flapper" archetype was prominent in the 20's, a huge decade for women's liberation. To be able to say, 'I'm beautiful with small hips and small breasts regardless of how easy it will be for me to have babies' may have been kind of a big deal. Also in the 20's and 30's, recession-era public works projects focused on improving society placed a new emphasis on the importance of vigorous physical exercise (this is the era when the Boy Scouts were growing, for example).
In the 40's and 50's, many of America's young men had fought in WWII—it makes sense that an ideal body form would more closely follow that of a more muscular man in good physical form, and when the country took a conservative turn extremely gendered roles in society were brought back with a vengeance, and once more women were expected to be homemakers, and look more curvaceous and "feminine" (as relates to their perceived ability to have and raise kids), while men were taking the burden of supporting their families on those nice, broad shoulders. This was also the first period where kids were raised with an official (i.e. based on government recommendations) food-group chart; nutrition meant eating a balanced diet of three meals a day and America was in a period of economic growth where this was possible for many families. It makes sense to me that there was a shift between looking 'healthy' pre-WWII and looking 'robust' in the baby boom.
I see the same thing happening in the 60's and early 70's, with women's liberation rising again and women becoming happy to be less "feminine" in their body shape, and it just wasn't "cool" for young men to strive for the ideal body of a soldier in the era of Vietnam. This was also the same period where "fat" became the enemy according to nutritionists, and in popular culture women were still supposed to take a passive role in life—thus more dieting, less exercising; more "cutting fat" and less working out.
And in the late 70's through the 90's I don't see women becoming "rounder" or "curvy" again, I see them becoming stronger. This was the era of people getting back to nature, crazes, for men and women, not just in various diets but in aerobics, jogging, hiking (or "nature walks"), yoga, and weight-lifting. Once again it was considered healthy and culturally appropriate for both men AND women to lead a vigorous and active life and to attain a physical balance between muscles and a normal amount of body fat.
Meanwhile, the 2000's, along with setbacks to women's rights and health issues as well as huge advances in plastic surgery, became an era of the "passive health" routine. As much as has been going around about living green, there are just as many "miracle cures." Women at all income stages are told to lose weight and/or become "healthy" a different way every year, whether it's cutting fat, cutting carbs, cutting sugar, eating a single "wonder food," drinking acai juice, eating "superfoods," whatever. And with the rise of obesity and diabetes, it's no wonder that most people mentally equate health with weight, rather than the health of the body in terms of "vigor."