when you smoke weed, your body uses up a shit load of all your vitamins and nutrients to process the THC , i dunno if that would contribute to muscles shrinking, but this process causes "the munchies", so your best bet would be to smoke up, get the chron munchies that you would usually put on weight with, but then go to the gym and turn that fat into muscle...i have tried getting fat but with my physical genetics its immpossible for me to get over 70 kgs, I duno what kgs is in lbs...You bloody Americans with your maveric ways...