Surely if you excersice alot .. Your muscle is weighing more than whats left on your body , try a bmi test on the internet (BMI= body mass index).. If you done mine.. i'm 13 stone (90kg) and i'm 14 but also the same height as you (5ft 11) and my bmi is 20 , it's healthy if you are 20-25. I play rugby on a friday after school , so i don't really bother if i have some flabby bits.. Haha! . just means i'm harder to take out