Your head is a 12-pound weight and in order for you to travel efficiently in the water and push through as small of a column of water as possible you have to get your head down in order to get your hips to come up. Your fulcrum, or balance point, of your body while attempting to be horizontal in the water is not at your belly button, it is directly under your lungs. You have more distance behind the fulcrum that in front of it. Therefore a 1-2" change in front of the fulcrum can make a 3-5" change behind the fulcrum.
Once proper head position is achieved, the second most important factor in improving your balance in the water is to get your lead arm into a position that is several inces below the water rather than at the surface. At first this might seem counter-intuitive, however the trade off here is what is important. Yes, you will have a small amount of added drag to get your arm lower, but by doing so your hips and legs go up. That is a good trade.