Aesthetics dont matter in boxing but what is your diet like? U need a high protein diet more high intensity resistance traing to put on lean muscle. And if you want to put on big muscle high protein and carb (low gi) diet and big lifting. Try the 5x5 ( 5 sets , of 5 reps of squats, deadlifts,shoulder press, chins and bench). Keep it heavy. Be carefull this takes a lot out of u and isnt really recommended for a fighter. Plus your only young. Only do it 3 days a week. Its good for packing muscle. Thats how i put on 2 stone in 2 years. Im sure some smartass on here will fault this advice but it worked for me mate, done properly. Your only young so dnt worry to much your muscles will develop in time.