A stone is heavy, and the sand weighty; literally, heaviness of a stone, weight of the sand. The substantives are more forcible than the corresponding adjectives would be: the versions rather weaken the form of the expression by rendering, Grave est saxum, etc.
The quality in the things mentioned is weight, heaviness, ponderosity; that is what we are bidden regard. A fool's wrath is heavier than them both. The ill temper and anger of a headstrong fool, which he vents on those about him, are harder to endure than any material weight is to carry.
Ecclesiasticus 22:15, "Sand and salt and a mass of iron are easier to bear than a man without understanding." The previous verse asks, "What is heavier than lead? and what is the name thereof [i.e. of the heavier thing], but a fool?"
Job speaks of his grief being heavier than the sand of the sea (Job 6:3).