Its much harder to sell products to people who are healthy and happy.  So companies choose to use images that are unrealistic in either direction.  They need to create that feeling of "there's something wrong with me, I need to buy something to fix it," or "I'm missing something...what can I buy?"  To be a "normal" model, you must be underweight.  To be a "plus sized model" you must be overweight.  To be a "fitness model" you must have low body fat.  None of the above three categories of wome's bodies are reproductively healthy (being in any of those three categories inhibits menstruation).  The only times we see a normal healthy woman used in advertising she's an actress, musician, or athelete.  Thank goodness there are some healthy role models.