Isn't it something like,
A heavy aircraft flying at a given speed must fly at a greater AoA than an identical aircraft that is lighter in weight flying at the same speed.
AoA at Va and stalling AoA are proportional for a given weight.
So for the heavier aircraft to maintain the same margin of AoA below stalling AoA (because it's already flying at an increased AoA for a given speed) The AoA must be reduced by increasing speed, in order to restore the margin of AoA below stalling AoA, which results in an increased Va for an increase in weight.
But I could be completely wrong and it could all be down to the lift fairies!