I am by far not the biggest person, but i have added 14 lbs of mass since i started training years ago (I was SCRAWNY), and i went through break periods (ie christmas) where i wasn't working out and eating bad. but i reccomend eating about a 2700 calorie diet, and alternate it out every two days with a 2200 caloire diet. also have a protien shake or bar, or even cottage cheese after a work out to give your body the protein it will need to build the muscle. always eat healthier foods, and you will start to gain some, but it wont happen over night. that my suggestion. good luck!