Breast Tissue and Fat Weigh the Same?

Fat volume for volume weighs about 80% of water. Breast tissue is closer water than it is to fat. Therefore it is my rough estimate that fat is probably somewhere in the 80-85% of the density of breast tissue.
I assume you are referring to liposuction when it comes to breast reduction. Also remember that the volume of aspirated material is not all fat. Depending on the situation, my measurements have shown that anywhere from 60-80% of the material removed is fat. Therefore you must aspirate about 650-875cc of tissue to equal 500cc of removed fat which will weigh about 400gms.
For example in the wikipedia patient below, I removed 900 cc of aspirate.

Unfortunately, there is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery.

I often suggest a patient's that they should have as much tissue removed as possible while  retaining proportionality with the remainder of your torso.  

Before undergoing the breast reduction procedure it is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.

Best wishes.

Breast tissue is generally more dense than fat and therefore heavier. It is really hard to predict how much breast tissue will weigh when removed and it is impossible to guarantee a postoperative breast size. My general approach is to ask a patient whether they want to be left on the full side or whether they want to be as small as possible while still having pretty breasts. There are some times that it is clear that more than 500 gms will be removed, particularly when the breasts are really large. If your breasts are smaller, it is not possible to predict and guarantee a resection weight of 500 grams.