I believe that a large factor in what becomes popular is how common or rare the thing is. There was a time when being plump was seen as a good thing. During times when life was more difficult, more people were thin because they had less money and less food to eat (perhaps did more manual labor that burned calories too). So, being plump indicated "success" at that time . . a person who could afford plenty to eat, and leisure.
Same with skin color: In previous generations in America, pale skin was highly prized. I noted that my grandmother wore a lot of clothes while working during summer in her garden. Among the reasons she did this (in addition to her and her peers not wanting to look "colored") may have been what you indicated, Blue: The laborer worked in the sun and got dark, while the wealthier business execs did not "have to" toil in the sun and were thus not tanned.
I think the current desire for being thin and fit is that so many -- in America anyway -- are neither. We value that which most do not have.