From the perspective of a flight control system design engineer without reference to any specific model, light weight / aft CG and heavy weight / fwd CG usually present the two extremes of airplane response characteristics that we must consider during design. Until recently augmented flight control laws for commercial transport airplanes have been designed without knowledge of the current weight and CG thus the closed loop response characteristics become a compromise between what is found at these two extremes of the weight / CG envelope. Heavy/fwd tends to present the most sluggish response and the highest required control forces as the airplane has its greatest inherent stability at this loading thus requiring more control input to maneuver. Light/aft the airplane is more responsive and has the lowest inherent stability thus can tend toward being overly sensitive.
FCeng84,
Judging from your location one could think you're involved in Boeing aircraft design? One day I might find myself in a 45 tonnes 737-600 and the next in a 75 tonnes 737-800. There sure is differences in how the two planes handle, a light 737 is not far from a C172. In turbulence you're thrown about like a leaf and you have to resist the temptation of your bum, as it wants you to kick some rudder to keep her straight.
Are the flight control systems identical on all 737 models (-600 to -900) or are parts (in the feel system) modified to account for different stability and inertia properties? Does the elevator feel computer have any idea of the current weight of the aircraft?