Take a look at Kelly McGillis in "Top Gun." What do you notice? This supposedly smokin' hot blond is a tad on the chubby side. Back in 1986, a few extra pounds (and eyebrow hairs) was apparently de rigueur.
I have three possible theories:
1) Nutrition Knowledge
There were fewer diets back in the day. And the ones I have heard about seem pretty primitive (though the classic grapefruit diet could probably give the more recent Hollywood cookie diet a run for its money). Regardless, I don't think this explanation is up to snuff since simply not eating was not invented yesterday.
Girls back in the day did not throw metal. It just wasn't done outside of some fringe groups. Plus there was no pilates, physique 57 or bar method. So, curves were more free form. Quite plausible as explanations go.
3) Breast Implants
To state the obvious--breast implants allow skinny girls to have large breasts. Before this invention came and took the world (or at least Southern California) by storm, only genetic anomalies could have this combination of traits. Now casting directors no longer have to decide between cleavage and a small waist. They can have it all and so they do. I have no scientific evidence, but this explanation actually seems the most convincing to me. The media might not have the power to completely shape our minds, souls and bodies but they make a good run at it.