How would overfull lungs (and therefore more positive buoyancy above the center of gravity) cause someone to be head heavy?
Balance underwater isn't really complicated. Basically, you are floating, without an attachment to the bottom. Therefore, you will rotate until you are in a position where the forces pushing you upward (lungs, exposure suit, and BC) are precisely balanced with the forces pushing you downward (weights and tank, most of the time). If you can align those forces so that they balance out in a horizontal position, you will remain horizontal when you are immobile. Otherwise, you are going to move, inexorably, into the position where those forces ARE balanced.
Some of the things that affect your position are variable. Your posture is a big part of this. Positioning body parts, incuding your head, arms and feet, can significantly affect your balance, and how they will do this will depend in part on whether your body parts are positive or negative. People with heavy neoprene and positive fins may find themselves with a lot of positive force low on their body, and tend to tip head down. People diving doubles may find that they have so much weight high on their body that they can't control the tendency to go head down.
The key to this is to put yourself in a useful diving position -- body fairly flat, legs out behind you, head up -- and see what happens if you don't move at all. That will give you some keys to where your combination of equipment and exposure protection puts your balance. If you are head heavy, you are simply going to have to move some ballast aft of your center of balance. Whether that's a weight belt, a DUI harness slung low, negative fins, or ankle weights, something is going to have to move south.
