Heya Creaze,
If I am right, it is as follows:
Vertical acceleration is induced by a Force. Given Newton's formula, we have F=m.a , or a=F/m
So if you would apply the same upward force (when pulling on the yoke) to the light and heavy aircraft, the upward acceleration of the lighter aircraft will be higher.
That means a heavier aircraft has a higher Va than a light aircraft
