Your max weight plane has a Va of Xias. Flying at that speed and weight, the difference between your alpha and the alpha to stall the wing is less than can be commanded by full up elevator. As your weight goes down with burned fuel, the same lift requires less alpha, therefore the margin expands. So, unless Va is consciously and correctly reduced by the pilot, he can damage the plane with sudden, full up elevator.
That is the most common explanation I hear, and I don't think it's right.
At the maximum travel, the airplane will always stall (with very few exceptions), regardless of airspeed or weight. Elevator more or less sets the angle of attack, and usually has the authority to go past the critical angle of attack.
That means, the maximum lift produced up full up elevator is the same regardless of airplane weight.