If I wear a size 40N bra and had a reduction to a D or DD cup size. How much tissue would be removed?

At
size 40 each 300 grams removed corresponds to 1 cup size change.If your measurement is accurate, you would
require 2,700 grams removed from each breast to achieve a DD.The largest breast I have ever performed was
a 44 M and I did remove approximately that amount to achieve her goal of a 42
DD.Best Wishes,Gary Horndeski, M.D.

There's no way to tell you how much weight needs to be removed to be a certain cup size. The surgery is not dictated by cup size. It Is dictated by shaped volume and proportion to the rest of your body. 

Without exam or photos-- impossible to evaluate. Best to schedule an appointment with a Board Certified Plastic Surgeon

 Congratulations on your decision to proceed with breast reduction surgery. You are correct in that you will be best off achieving a long-term stable weight prior to proceeding. They will be well worth your effort; breast reduction surgery tends to be one of the most patient pleasing operations we perform.  Well experienced board-certified plastic surgeons will be able to give you a much better “estimate” in person than with online consultation.  You will find that there is no accurate/reliable correlation between amount of tissue removed and specific cup sizes achieved. Remember, as you communicate your goals with your plastic surgeon, that a discussion of cup size can be confusing and/or a source of miscommunication. In my practice, I prefer the use of goal pictures to help with this communication process. Generally speaking, I suggest patients undergoing breast reduction surgery have enough tissue removed to help alleviate/decrease  symptoms they are experiencing,  while preserving enough tissue to feel comfortable about the upper and lower body  proportionality.
Best wishes.

It is impossible to tell you how much tissue can be removed without an exam in person. I agree, lose the weight first and then undergo the procedure.

Insurance coverage for breast reductions defies logic and reason.  Insurance companies frequently deny breast reduction surgery if your BMI is greater than 30.  Know the restrictions placed by your insurance company and seek a qualified plastic surgeon for recommendations. 

Thank you for your question. Women who have large breasts may experience a variety of problems from the weight and size of their breasts, such as back, neck and shoulder pain and skin irritation. Many women who suffer from back pain due to the size of their breast find that the pain is unrelieved by over the counter medications, support garments, bras, and physical therapy. Some women even develop significant arthritic changes in their neck and upper back. They often find that their persistent symptoms cause a significant restriction of daily activities. Occasionally, women with large breast, suffer from non-seasonal rashes underneath their breast that are unresponsive to local hygiene and topical creams and powders. Women with large breast may notice shoulder grooving with skin irritation due to their bra straps and support garments. They may also notice pain and numbness in their arms and hands. Breast reduction is usually performed for relief of these symptoms rather than to enhance the appearance of the breasts. Breast reduction surgery removes excess breast fat, glandular tissue and skin to achieve a breast size in proportion with your body and to alleviate the discomfort associated with overly large breasts. The best candidates are those who are mature enough to understand the procedure and have realistic expectations about the results. There are a variety of different surgical techniques used to reduce and reshape the female breast. I recommend that you make an appointment with a board certified plastic surgeon to discuss your surgical options.

No posted photo so my over the internet guess is 1,000 grams per side at least. But only in person examination allows a definitive answer...