The way in which Christ is in this sacrament
Objection 1. It seems that the whole dimensive quantity of Christ's body is not in this sacrament. For it was said (3) that Christ's entire body is contained under every part of the consecrated host. But no dimensive quantity is contained entirely in any whole, and in its every part. Therefore it is impossible for the entire dimensive quantity of Christ's body to be there.
Objection 2. Further, it is impossible for two dimensive quantities to be together, even though one be separate from its subject, and the other in a natural body, as is clear from the Philosopher (Metaph. iii). But the dimensive quantity of the bread remains in this sacrament, as is evident to our senses. Consequently, the dimensive quantity of Christ's body is not there.
Objection 3. Further, if two unequal dimensive quantities be set side by side, the greater will overlap the lesser. But the dimensive quantity of Christ's body is considerably larger than the dimensive quantity of the consecrated host according to every dimension. Therefore, if the dimensive quantity of Christ's body be in this sacrament together with the dimensive quantity of the host, the dimensive quantity of Christ's body is extended beyond the quantity of the host, which nevertheless is not without the substance of Christ's body. Therefore, the substance of Christ's body will be in this sacrament even outside the species of the bread, which is unreasonable, since the substance of Christ's body is in this sacrament, only by the consecration of the bread, as stated above (Article 2). Consequently, it is impossible for the whole dimensive quantity of Christ's body to be in this sacrament.
On the contrary, The existence of the dimensive quantity of any body cannot be separated from the existence of its substance. But in this sacrament the entire substance of Christ's body is present, as stated above (1,3). Therefore the entire dimensive quantity of Christ's body is in this sacrament.
I answer that, As stated above (Article 1), any part of Christ is in this sacrament in two ways: in one way, by the power of the sacrament; in another, from real concomitance. By the power of the sacrament the dimensive quantity of Christ's body is not in this sacrament; for, by the power of the sacrament that is present in this sacrament, whereat the conversion is terminated. But the conversion which takes place in this sacrament is terminated directly at the substance of Christ's body, and not at its dimensions; which is evident from the fact that the dimensive quantity of the bread remains after the consecration, while only the substance of the bread passes away.
Nevertheless, since the substance of Christ's body is not really deprived of its dimensive quantity and its other accidents, hence it comes that by reason of real concomitance the whole dimensive quantity of Christ's body and all its other accidents are in this sacrament.
Reply to Objection 1. The manner of being of every thing is determined by what belongs to it of itself, and not according to what is coupled accidentally with it: thus an object is present to the sight, according as it is white, and not according as it is sweet, although the same object may be both white and sweet; hence sweetness is in the sight after the manner of whiteness, and not after that of sweetness. Since, then, the substance of Christ's body is present on the altar by the power of this sacrament, while its dimensive quantity is there concomitantly and as it were accidentally, therefore the dimensive quantity of Christ's body is in this sacrament, not according to its proper manner (namely, that the whole is in the whole, and the individual parts in individual parts), but after the manner of substance, whose nature is for the whole to be in the whole, and the whole in every part.
Reply to Objection 2. Two dimensive quantities cannot naturally be in the same subject at the same time, so that each be there according to the proper manner of dimensive quantity. But in this sacrament the dimensive quantity of the bread is there after its proper manner, that is, according to commensuration: not so the dimensive quantity of Christ's body, for that is there after the manner of substance, as stated above (ad 1).
Reply to Objection 3. The dimensive quantity of Christ's body is in this sacrament not by way of commensuration, which is proper to quantity, and to which it belongs for the greater to be extended beyond the lesser; but in the way mentioned above (ad 1,2).
Source: Summa Theologica (New Advent)