...it's all well and good trying to define mass - but without knowledge of the underlaying structure all your managing to achieve is undermining your own learning by just focusing on the muscle mass...
There's a number of layers you need to consider when creating figures, the first is the skeletal structure, then the muscle masses, then the fat masses, then the skin and hair, and finally the clothing.
Look deeper into what you've done here... and not only focus on the cranial mass, but also the spine, the clavicles, the thoracic cavity, sturnum and diaphragm, the pevlic girdle, the patellas, the femurs, tibia and fibia and of course - the feet and not forgetting the arms and hands and fingers... build the skeletal structure, or at very least to key components where the form (that you've nearly got right) hangs. Then and only then can you build on that with the muscle mass.
You'll discover immediate improvements if you understand from within and there's plenty of reference for anatomy knocking around the interweb.
Eventually once you get to know the underlying form you can deconstruct it to just a few flicks of the wrist and then you can jump straight to the masses... more experienced artists just 'know' what goes where and how to place mass instinctively... but even they had to learn from within first.