When you work out using only your body weight, also known as calisthenic exercise, you work out all the smaller muscles in your body as well. There are lots of muscles which will benefit from body weight exercises - you just need to perform them!
Body weight exercises are also very natural for your body. Take push ups and sit ups for example, you are working out lots of muscles doing these exercises. They are natural movements and can improve your physique.
Calisthenics are a lot more difficult then they look, try them and see for yourself!
