What can I do if the surgeon didn't remove enough breast tissue for insurance?

The weight requirement that most insurance companies use, to which you refer, is based on the Schnur sliding scale.  It is based on a patient's body surface area, and is used by insurance companies mostly to deny authorization or payment for breast reduction surgery.  This scale is widely used by insurance companies, but is not based on any scientific evidence.  These weight requirements will not predict whether or not a patient will have functional relief of symptoms following breast reduction surgery.  For these reasons, the American Society of Plastic Surgeons is opposed to the use of this scale by insurance companies.  How much tissue removed is a judgement call.  It is often based on what looks good relative to the rest of your body, safety, and any pre-operative discussions about expected post-operative size.  Sometimes you don't know beforehand whether the weight requirement will be met.  My advice would be to discuss these issues with your plastic surgeon and ask him or her to write an appeal letter.