there is a difference between lean muscle mass which doesn't weigh much and large muscle mass.
the difference is you will need a small shirt to show off your body while someone with large muscle mass looks big no matter what they wear.
I agree with nelson.. focus on gaining mass SLOWLY. not fast. you do not need steroids at all right now. also don't forget your legs!! not sure if your legs are toothpicks or what (they have to be for you to be your height and weight), but you need a strong trunk so don't neglect them for abs