When St Thomas speaks of quantitas dimensiva, he means 'visible bulk.' Of invisible bulk he took no note at all. He says (Chap. LXIII, not translated):

"If the substance of bread were resolved into its prime corporeal elements, this change would necessarily be perceptible to sense, since corporeal elements are sensible." By 'corporeal elements' he meant fire, air, earth, and water. The schoolmen herein followed Aristotle; Aristotle followed Plato; and Plato had a singular dislike for Democritus, the author of the atomic theory. Compare however the elemental trigona of Timaeus, 53 sq., which triangular atoms Plato expressly declares to be invisible. Every chemist and electrician now recognises that visible bulk is not of the essence of material substance; and the question stands open, whether the ultimate elements of such substance have any bulk, even invisible, -- whether they are extended at all. But every body is extended, still more every living body, in its natural state. We say then that the Body of Christ is not in the Sacrament of the Altar in its natural state.

Of God and His Creatures: 4.67