Some insurance companies, like yours, have a flat requirement of 500 grams for "approval." That does not take into account whether you are 5'5", 5'7" , 5'11' or what your body weight is. From what you tell us, your breasts are not that big (between D and DD. medium frame)
The problem for your surgeon is that, during surgery, he/she may find that your shape and size would be great with a 400 gram reduction. Your surgeon is then faced with a terrible option: leave it at 400 grams (and trigger nonpayment by the insurance company), but give you what he/she thinks is a great size and shape. The other option is to try to please the insurance company by getting to 500 grams and risking giving you breasts that are too small or with a poor shape.
I suggest you go your kitchen/grocery store and figure out exactly what 500 grams (slightly over one pound) looks like. You can then try to imagine removing 500 grams from your own breasts and see if that is still a desirable size. Ask your surgeon to help you in this decision ahead of time.
The last thing you want is a surgeon that feels obligated to cut more out than is indicated simply to comply with an insurance company policy.