1870s: In the late 19th century, being overweight was a sign of wealth, since it suggested one had the means to feast.
This photo shows the members of a "Fat Man's Club" in Connecticut in 1894. The club was founded in 1866, and members had to weigh at least 200 pounds. The idea became popular and spread across the US.
1930s: By this time, excess weight started to get associated with lower classes. Hollywood actors — who had to be fairly fit to look good on camera — started to represent a new ideal.
A relatively average-sized, generally (though not extremely) muscular body became the ideal.
1960s: By the late '60s, a counter-cultural wave had begun to reject the look associated with the corporate mainstream.
Rock stars with long hair and skinny bodies started to represent the ideal.
1980s: In the '80s, a more hyper-masculine, gym-built form started to gain popularity.
Bodybuilding went mainstream and action stars rocked huge muscles.
1990s: By the '90s, hyper-masculinity went back out of style, and the everyman came in.
A more blue-collar, working man's look — one that didn't require pumping iron — became the look men tried to cultivate.
The ideal now is still relatively similar to that of the '90s — lean yet muscular.
But the ideal will continue to transform, often in response to the state of the world.
And of course, that ideal usually differs from the average real body, both for men and women.
In the future, new technology may give us more control over how we look. Who knows what will be in favor then?