I am working out,lifting high wieghts low reps,and am getting stronger,but am not getting bigger. why?

Maybe you started out with a higher body fat percentage and you lost fat while gaining muscle so your still around the same weight, but more muscle. Also you might not be taking enough calories or protein to help you put on mass. You have to take in a certain amount of calories per day to gain weight. You could try eating more (healthy foods) like tuna, skinless chicken breast, protein shakes, sweet potatoes and whole grain pasta for carbs, egg whites, cottage cheese and nuts.

getting bigger requires a higher protein diet.

you need to eat to get big