Hello ace
This is something Dr Bermant would be best placed to answer, so I would post this question to him on the main gynaecomastia talk board.
My version is this:
If a guy has excess fat and glandular tissue in the chest area, this can be surgically removed by excision and liposuction to 'normalise' the shape of this area.
Now that the tissue has been removed, there is still a layer of fat, but it is in proportion to the rest of the body.
It's the surgeons task to remove the right amount of fat in line with how fat or thin the patient is. If too much fat is removed, then the chest might look abnormally flat in relation to the rest of the body. In this case, even if the the guy concerned stuffed his face with ten big macs every day and put on significant weight, the chest area will put on weight, but proportionately less than the rest of the body, so will still look too flat in relation to the rest of the body.
Fat cells have an amazing ability to expand in size. Take identical twins. If one eats loads of fatty or high carb foods and the other has a normal healthy diet, the one former will put on weight. No matter how much weight he puts on, be it half a stone or ten stones, each twin will have an identical number of fat cells. Only the fat twin's fat cells will be much, much larger than the thin twin's fat cells.
So, there is no such thing as 'new' fat. All that happens is that the existing fat cells swell up and make the person concerned look like a right fat biffer.
Does that help? :-/