To gain a 1 lbs of weight is extreamly easy to do, but to gain 1 lbs of fat, it may take more days, for the average person, it takes 2000 calories a day to live, if you eat over that....example, 3000 calories per day for 4 days, that would be an extra 4000 calories after 4 days and 3500 caloris = 1 lbs of fat. so you would gain lil bit more than 1 pound of fat. (a bigger person needs more calories a day or else they would start losing fat)