Many men undergoing male breast reduction surgery to treat gyncecomastia are overweight. It is healthier if a patient loses weight on their own. Surgery cannot make someone look as good as they would look with their own weight loss.
But even with weight loss, often some excess chest fat remains, and so overweight men often still want to have male breast reduction surgery after they have lost weight.
Finally, it is important that when men are overweight that only enough is removed from their chest to make it proportional with the abdomen. There is a tendency among patients and plastic surgeons to remove as much as possible, but when the chest is made much less prominent than the abdomen, this does not look good. And this can become even worse if the patient loses weight after their gynecomastia surgery.