I'm not an expert in psysiology, but I seem to remember from my old workout days that it takes years to build appreciable muscle weight. Muscle weight gains of 5 pounds a year would be expected from lifting weights maybe 3 times a week for a whole year, if you're a young male. Females don't add muscle weight like males do. It's all in the hormones, and genetics play a factor, also. The Biggest Loser contestants stand to lose a lot of water weight and fat, without any appreciable gain of muscle weight.