Fashion models are significantly taller and slimmer than most women. The fashion model's length and simplicity of form help display clothing with a dramatic affect. Style lines, asymmetry, and silhouettes of garments are exaggerated when displayed on an elongated scale. This elongated scale helps to portray a fantasy. These garments are then edited into the real world to be enjoyed by a wider range of people.
The image on the left is an average woman. The image on the right is photoshopped to represent an exaggerated fashion croquis or template that is 9 heads tall from the top of head to the ankles. This proportion is common to fashion sketching.
Drawing the fashion figure with 9 heads proportion—Part 1 goes more into depth about the proportions of the fashion figure. This is the place to start if you're interested in fashion sketching.