Okay, here is my shot at answering your questions, take it for what it is worth, I am no expert but here is my best common sense guess:
Let’s think about your bicep. It runs pretty much from your elbow up to your shoulder. Relaxed (stretched) it is longer and flatter. Contracted (bunched) it is shortened, but raises higher. Basically, the mass or density doesn’t change, just where the fibers are located.
Now if you think about a ‘pumped’ muscle, it is full of blood. So yes, it is denser, more weight in the same volume.
For your second question, I think if you were to try to build these muscles, injuries would come before visable results. These muscles are not designed for strength, they are designed for repetitive motion, such as chewing, or typing. I doubt the fiber type has the capability for much growth.
On the “jaw and finger” issue, that is what gives support to the idea that your body grows as a whole. The muscles in my jaw are more prominant than they were when I was much smaller. It isn’t like I was chewing weights over the years.
In fact, MOST bodybuilders have larger facial muscles the bigger they get. Dexter Jackson is one whose face has changed drastically from when he first started competing. It gets wider along with his neck.
As far as whether you “train” them directly for growth, I agree that you would probably either see fatigue or injury issues before you actually just built up your jaw muscles alone. It sounds like a TMD waiting to happen.