Chubby women used to be regarded as more desirable as they were more likely to survive childbirth. The fat is there (up to a point) to increase survival rates in adverse conditions. So it wasn't just chubby women, but a bit chubbyness in general which is/was a good thing.
The media can take a significant portion of the blame. Having women chase something unobtainable is a good way to keep selling them stuff that they don't need.
I suspect the upper classes also had something to do with the idea of chubby women being less attractive, if one considers the amount of stupid practices involving womens' looks. In Japan, binding womens' feet so they looked smaller. Corsets also. I can't think of others at present. The upper classes would have been less concerned with survival rates because their environment would have been to their advantage to begin with (better sanitation, food, etc). Then along comes the middle class, and wannabe middle class and wannabe upper class... suddenly most people want something pretty similar.
The person who said that wealthy women were generally fat is wrong AFAIK - a lot of food that typically makes you fat has always been cheaper than good quality food. A poorer family would try to make the most of an animal they've bought/raised for food. A wealthier family may also eat poorly so they end up being fatter, but they're more likely to go for more select cuts of meat and make less good use of an animal raised for slaughter.
The 'chubby' picture is debatable. Perhaps if the same woman had her picture taken standing straight and the shot taken from the side or straight on, rather than using her arm to potentially hide 'chubbyness'.
Personally I want a woman who takes pride in her figure and keeps in shape physically (though I want a woman to have woman's legs, not a teenager's legs). Now I just need to keep up my end of the bargain
