Why so many post like this... "how can I fill out my inner pecs/ outer pecs/ lower pecs"? Think about this... if someone were to say on this board:
Muscle shape is NOT genetic.
You can alter the shape of a muscle through training.
...or other such statements, wouldn't they be rediculed and laughed at by anyone who has any knowledge of biology? Yet why do we persist in answering these questions? Either muscle shape is genetic or it isn't. If it is, then you cannot force part of a muscle to develop faster than another, nor can you isolate certain parts of a muscle by changing the grip or angle of a lift. If that is the case, then all you can do is continue to overload that muscle, and all portions of it should grow at a similar rate. It could be that the inner insertions in the pecs are much smaller in thinckness than the meaty part of the muscle, so while a 100% increase in overall pec mass might account 2 extra inches of thickness on the thicker part of the muscle, a 100% incease on this person's inner chest, because it was 1/5th of the thickness of ther belly of the muscle to begin with, so the depth of this part of the muscle has increased by less than 1/2". Or perhaps he is missing that deep cut down the middle that he associates with inner chest development, but it is only body fat or extra subcontanious water that is obscuring it. My advice, continue to develop your chest and clean your diet up a bit, and achieve what you desire. Just my 2 cc's...