The effect of gust on an airframe isn't just a straightforward question of weight and aerodynamics. The mass distribution in the airframe is also important. The airframe structural response to a given gust is a function of the strutural mode shapes, which change depending on how the mass is distributed. When loads analysis is done for a new aircraft type, different combinations of weight distributions are analyzed with different gust intensities and wave lengths. The most critical loads for a piece of a structure will occur at a combination where the gust frequency is most exciting the structural mode shape. Not all parts of the aircraft will have critical loads at the same combinations of weight and gust. So, for example, you can imagine two aircraft with same total weight, however one has a lot of cargo and little fuel in the wings, the other little cargo, but a lot of fuel in the wings. Even though the total aircraft weight is the same, the loads produced on the wings will be very different due to the mass of the fuel.