These women's weight gain ads seem funny because they are so out of step with most women's concerns today. But what changed? My guess is that the whole weight-gain thing really was about larger breasts. If you wanted more cleavage, you had to gain weight. But breast implants changed that. Now one can have an improbable rack while still starving. So while breast implants are a positive in terms of empowering women to have control over their body, they have eliminated an important counter-balance to this crazy pressure on skinny-ness.
Disclosure: On a scale from 1=Kate Moss to 10=Rubens paintings, my preferences definitely are in the higher numbers, so I am not without bias. I also have a daughter who wastes way too much of her life worrying whether her body properly meets societal expectations for fat content.