I think the most telling sentence is

"The report shows that women’s attitudes to slimming over the last 50 years have changed with their figures".

It's a matter of relativity. When you're a very big person, being moderately sized is ideal in your eyes as a realistic goal. When you're already moderately sized, being very slim is ideal in your eyes as a realistic goal. Therefore since womens sizes have generally gotten bigger, it makes sense when you survey them, that most will want to be a moderate size. Womens attitudes have in fact not changed over the years, all women still want to lose the same amount of weight, it's just that they are bigger now, so when they realistically reflect on what size they would want to achieve, then it's going to be a bigger size.