The image isn't photoshopped though, according to the ASA's investigation. Her body shape has not been changed. The thigh nearer the camera is pressing down on the block she is sitting on so obviously looks wider than the other one that is not, with muscle tensed due to her pose, and the perspective effect of being further away.
Looking at this picture it really doesn't look fake in terms of her proportions and the investigation established that it isn't. The only alteration they made was to make the actual jeans look longer (presumably too short for her long legs), not to change her body.
The other photos a PP posted are clearly badly doctored so they would have been a good reason to demand the ad is removed, aside from them looking totally weird! But the ASA state they banned the ad due to this specific photo of her sitting on the wooden block in which they state her body shape hasn't been altered and their reasoning is that - while they admit she isn't unhealthily thin and this is just her build - they find it unacceptable that the photo accentuates her tall and slim build because this "makes her look unhealthy" in their opinion.
That is their reasoning, quoted in the article: that they think her body is unacceptable and, though healthy, they think she "looks unhealthy" due to being tall and slim so photos of her from certain angles shouldn't be allowed to be used in advertising. That's disgraceful.