If you look at urological sites for female-to-male reconstructive surgery the procedures often involve significant skin/tissue grafts and/or prostheses. See, e.g. this interesting and somewhat graphic site.
Penis enlargement (see this site) for cosmetic purposes often entails less drastic measures, such as detaching the ligaments that keep part of the shaft inside the body or grafting (relatively small areas of) tissues to widen the penis. Weights are used to keep scar tissue from drawing the penis back into the body after surgery.
The seriousness of sex re-assignment surgery would seem to make it inappropriate
for a patient who wants a cosmetic versus a morphological change.
There is a huge urological literature on these topics but I think the above give an indication of why the two situations might call for different sorts of surgery.