Objection 1. It would seem that the pains of Purgatory do not surpass all the temporal pains of this life. Because the more passive a thing is the more it suffers if it has the sense of being hurt. Now the body is more passive than the separate soul, both because it has contrariety to a fiery agent, and because it has matter which is susceptive of the agent's quality: and this cannot be said of the soul. Therefore the pain which the body suffers in this world is greater than the pain whereby the soul is cleansed after this life.
Objection 2. Further, the pains of Purgatory are directly ordained against venial sins. Now since venial sins are the least grievous, the lightest punishment is due to them, if the measure of the stripes is according to the measure of the fault. Therefore the pain of Purgatory is the lightest of all.
Objection 3. Further, since the debt of punishment is an effect of sin, it does not increase unless the sin increases. Now sin cannot increase in one whose sin is already remitted. Therefore if a mortal sin has been remitted in a man who has not fully paid the debt of punishment, this debt does not increase when he dies. But while he lived he was not in debt to the extent of the most grievous punishment. Therefore the pain that he will suffer after this life will not be more grievous to him than all other pains of this life.
On the contrary, Augustine says in a sermon (xli De Sanctis): "This fire of Purgatory will be more severe than any pain that can be felt, seen or conceived in this world."
Further, the more universal a pain is the greater it is. Now the whole separate soul is punished, since it is simple: which is not the case with the body. Therefore this, being the punishment of the separate soul, is greater than any pain suffered by the body.
I answer that, In Purgatory there will be a twofold pain; one will be the pain of loss, namely the delay of the divine vision, and the pain of sense, namely punishment by corporeal fire. With regard to both the least pain of Purgatory surpasses the greatest pain of this life. For the more a thing is desired the more painful is its absence. And since after this life the holy souls desire the Sovereign Good with the most intense longing--both because their longing is not held back by the weight of the body, and because, had there been no obstacle, they would already have gained the goal of enjoying the Sovereign Good--it follows that they grieve exceedingly for their delay. Again, since pain is not hurt, but the sense of hurt, the more sensitive a thing is, the greater the pain caused by that which hurts it: wherefore hurts inflicted on the more sensible parts cause the greatest pain. And, because all bodily sensation is from the soul, it follows of necessity that the soul feels the greatest pain when a hurt is inflicted on the soul itself. That the soul suffers pain from the bodily fire is at present taken for granted, for we shall treat of this matter further on [Cf. Supplement, 70, 3]. Therefore it follows that the pain of Purgatory, both of loss and of sense, surpasses all the pains of this life.
Some, however, prove this from the fact that the whole soul is punished, and not the body. But this is to no purpose, since in that case the punishment of the damned would be milder after the resurrection than before, which is false.
Reply to Objection 1. Although the soul is less passive than the body, it is more cognizant of actual suffering [passionis]: and where the sense of suffering is greater, there is the greater pain, though the suffering be less.
Reply to Objection 2. The severity of that punishment is not so much a consequence of the degree of sin, as of the disposition of the person punished, because the same sin is more severely punished then than now. Even so a person who has a better temperament is punished more severely by the same sentence than another; and yet the judge acts justly in condemning both for the same crimes to the same punishment.
This suffices for the Reply to the Third Objection.
Source: Summa Theologica (New Advent)