I'll try to give you correct and complete answers to your question
Concerning women, the answer is this :
Until the twentieth century, having enough to eat wasn't evident every year or every month in Western civilization and much children (and women) died at birth/ giving birth or very young. Fertility has been a high necessity for mankind during thousands of years. People didn't live very old and had to get a lot of children to sustain them later: there were no pensions, no health insurance plans.
All that was their children's' responsibility, and in a society based on agriculture many hands were needed. Having many children was their insurance and pension plan, their wealth. (family life & relations are still like that in most developing countries. Once they get social security, overpopulation can end)
"Rounded" women had fat reserves to survive a period of food shortage and still bear children. Large breasts were perceived as (potentially) full of milk for the baby. That was the kind of female body a man desiring offspring would look for. During the fifties, fat babies were still very much in, as were full breasted Mae West style women: There had been two World Wars and poverty from the 1930 crisis.
At this moment, youth is idealized and the slender woman looks more like an adolescent girl than a full grown woman.
The David by Michelangelo was one of his first works (himself thus being young) and he was inspired by (Roman copies of) Greek statues.
Later in his life Michelangelo will represent more solid and muscled men: link
link (one should mention too that Michelangelo was bisexual, probably with homosexual prevalence)
The male body shown is an American stereotype that originated in Hollywood but muscled men can be found in abundance in art history: men had to protect the family, to fight, to hunt.
Thus, yes, body ideals changed and they changed under influence of civilization, economics and history.
Probably because men have sometimes needed women to be a certain way, to gratify their own peculiar tendencies. Women are more intuitive, emotional creatures who tend not to get so messed up. They know a hot body when they see it, and there isn't too much variety in what a hot body is. A hundred years ago, a respectable man might not even want a sexy wife, because she would be too much for him to handle. Nowadays, people are getting more and more sexually-liberated, and nearly everyone under 50 can agree, the hotter, the wilder, the sexier, the better!
the way things are portrayed in art isn't necessarily what the ideal body was. it's just the way things were.
men have always been warriors and laborers. therefore, they have always been fit because their work makes them fit.
women, however, worked in the house. their work may have still been stressful, but it wasn't the kind that burns a lotta calories. wealthy women would really only have to raise children. and there were no stairmasters or weight watcher programs. also, women used to marry very young, and there pretty much was no birth control. we all know what having children does to the body
The ideal male's body probably won't ever change because it is not only the body that is attractive, but the concept of being strong. A strong man can offer you comfort, protection, better sex because they are in better shape, and they are more athletic.
As for the ideal woman, its just all preference. We live in a much more sex driven society today, and part of sex is being sexy. And having a nice toned body, with a nice ass, and good t*ts is a biiiiiiiig part of being sexy.
I think its because the family unit in society has changed. When the goal of a relationship was to marry and have children, a woman whose body was more "healthy" looking, and curvacious (great for breeding..hate to put it that way but...so it is) was very attractive.
Nowadays...having babies isn't the primary goal...its not a given that we're going to pump out as many as our bodies will allow and stay at home with the kids, many more women are pursuing careers and make that a priority over starting a family and many people don't want to have kids at all. So having the body for it isn't really as important as it used to be.
But then and still now...generally speaking, men are looked at as the caregivers, the strong one, the protective one, etc. Unlike the role of women, the role of men and how we perceive them hasn't changed too much over the years.
at many times through history the ideals of what a man and women should be has changed. Often, particularly among nobel europeans, the ideal man was very effeminate, rather then strong or physically tough.
this is a fascinating subject. I did a project on it once, I think part of why the standards of women have changed so drastically is because the their roles have changed. their role used to be to bear children and to stay in the home. a woman built like in the portrait would be good for bearing sons. but now women are more companions, they are active and out of the house.