cos when you are lighter you can reach the max load factor of the aircraft at a slower speed. If you produce X amount of lift on a plane weighing 2000lb, then produce the same X amount of lift on the same aircraft but this time only weighing 1750lb then the loading of the airframe will be greater. Because Va normally relates to a maximum loading, then in order to not exceed this loading for the lighter aircraft they limit the airspeed, and hence the maximum amount of lift it produces and keeps the airframe within loading limits