I am currently in the process of losing weight and building my upper body muscles in preparation for top surgery (end of this year). My doctor is encouraging both, and explained why: Getting to a healthy weight for your size will help in overall recovery. The healthier you are, the more energy gets spent on the healing process, making it go quicker and easier. Building the muscles provides even more support for the healing process (more muscle=more blood flow, which is what feeds and cleanses the area that is healing), but it also provides a canvas for the surgeon to work with. Everyone's muscles are shaped a little differently and bulk up in different ways. With well-developed muscles, the surgeon knows where best to make incisions, place nipples, and contour to create the most realistic male chest.
Some surgeons also have two options for the facility used. For example, one surgeon I know of can perform the DI mastectomy in his surgery center as an outpatient procedure (saving money, even for those of us who are lucky and have health insurance helping cover the costs), but only if you have a BMI of 32 or less. Larger (like I currently am!), and you have to be admitted to the big hospital.