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.