I got very, very Ill two years ago - lost nearly 50% of my body weight as a result. Gained a healthy amount of weight back so I am no longer a waif. When I shrunk down and wasted from disease, damn, huge tuned into unbelievable because of loss of fat pad at the base of the shaft. Girth stayed right where it was but apparently I had another inch *hiding* in there.
So do not fear dropping weight, it will not affect the body of the penile shaft one bit. It could push you outwards of the normal spectrum though and give you that lean, bedroom Adonis look. ;> No worries, eh?