At the other end of the scale (pardon the pun) obese or overweight men are portrayed more positively in the media than women of same BMI...
This is particularly prevalent in American TV shows, even the animated ones! Family Guy, American Dad, The Simpsons, notice the male characters in these shows tip the scale considerably more than the women in these shows?
There's also Man Vs Food, a show that probably wouldn't exist at all if a woman had pitched it. Nobody reduces male actors as merely 'plus size role models' like they would brilliant and hillarious female actors such as Rebel Wilson or Melissa McCarthy.
With men, heavyness is powerful, strong, or even jolly (Santa?), lightweight is athletic. Women however are seen to be either 'promoting obesity' or 'promoting annorexia' so at either end of the scale, women's bodies are always viewed to be making a statement.
As a society, we need to remove gender from the equation entirely. If seeing underweight or overweight women in the media is said to be sending out the wrong messages and promoting unhealthy body images, then surely so are the men from either end of the scale?
Or alternatively if we can easily accept that men come in all shapes and sizes, without this impacting on public health, then we as a nation need to reach this level of acceptance with women's bodies too.