I really wish I would have gotten in on this thread a long long long time ago. I am in the Air Force and have been for 14 years one of my primary jobs is that of a load planner. The window in which a center of gravity can be in order for an aircraft to be flyable becomes smaller the heavier the aircraft is and larger the lighter the load is. Pictures are so much easier than trying to explain in terms everyone will understand. Suffice to say the lighter it is the easier to balance.