I agree with Realgains here, After working out with flat, incline, decline benches, I notice a huge difference in pumps when doing the decline.
Although I do believe that you can alter the size of a particular part of a muscle, depending on the form used during an exercise I do agree with macho21 and Mischief on the shape of your muscle, they are what they are, that's what makes us all different.
To clarify what I am talking about as far as altering the size of a particular part of a muscle. I do very wide grip on the decline so that to work the lower outer corners of the chest, to give it a more square shape. I also do them close grip to limit the amount the chest muscles gap in the middle of the chest.
I probably just confused the hell out of everyone.