Given a particular load factor limit I've always thought Va has to be proportional weight (unless it's fast enough at a lower weight that a higher weight limit isn't worth bothering with).
My logic is that at a lighter weight a force applied to the airframe by control surface deflection will produce a greater acceleration than the same force applied to a heavier fuselage. Greater acceleration equates to a greater LF. Ergo, reduce Va at lower weights to limit the maximum acceleration ability.