Objection 1. It would seem that the temporal punishment, the debt of which remains after Penance, is not imposed according to the degree of fault. For it is imposed according to the degree of pleasure derived from the sin, as appears from Apocalypse 18:7: "As much as she hath glorified herself and lived in delicacies, so much torment and sorrow give ye her." Yet sometimes where there is greater pleasure, there is less fault, since "carnal sins, which afford more pleasure than spiritual sins, are less guilty," according to Gregory (Moral. xxxiii, 2). Therefore the punishment is not imposed according to the degree of fault.
Objection 2. Further, in the New Law one is bound to punishment for mortal sins, in the same way as in the Old Law. Now in the Old Law the punishment for sin was due to last seven days, in other words, they had to remain unclean seven days for one mortal sin. Since therefore, in the New Testament, a punishment of seven years is imposed for one mortal sin, it seems that the quantity of the punishment does not answer to the degree of fault.
Objection 3. Further, the sin of murder in a layman is more grievous than that of fornication in a priest, because the circumstance which is taken from the species of a sin, is more aggravating than that which is taken from the person of the sinner. Now a punishment of seven years' duration is appointed for a layman guilty of murder, while for fornication a priest is punished for ten years, according to Can. Presbyter, Dist. lxxxii. Therefore punishment is not imposed according to the degree of fault.
Objection 4. Further, a sin committed against the very body of Christ is most grievous, because the greater the person sinned against, the more grievous the sin. Now for spilling the blood of Christ in the sacrament of the altar a punishment of forty days or a little more is enjoined, while a punishment of seven years is prescribed for fornication, according to the Canons (Can. Presbyter, Dist. lxxxii). Therefore the quantity of the punishment does not answer to the degree of fault.
On the contrary, It is written (Isaiah 27:8): "In measure against measure, when it shall be cast off, thou shalt judge it." Therefore the quantity of punishment adjudicated for sin answers the degree of fault.
Further, man is reduced to the equality of justice by the punishment inflicted on him. But this would not be so if the quantity of the fault and of the punishment did not mutually correspond. Therefore one answers to the other.
I answer that, After the forgiveness of sin, a punishment is required for two reasons, viz. to pay the debt, and to afford a remedy. Hence the punishment may be imposed in consideration of two things. First, in consideration of the debt, and in this way the quantity of the punishment corresponds radically to the quantity of the fault, before anything of the latter is forgiven: yet the more there is remitted by the first of those things which are of a nature to remit punishment, the less there remains to be remitted or paid by the other, because the more contrition remits of the punishment, the less there remains to be remitted by confession. Secondly, in consideration of the remedy, either as regards the one who sinned, or as regards others: and thus sometimes a greater punishment is enjoined for a lesser sin; either because one man's sin is more difficult to resist than another's (thus a heavier punishment is imposed on a young man for fornication, than on an old man, though the former's sin be less grievous), or because one man's sin; for instance, a priest's, is more dangerous to others, than another's sin, or because the people are more prone to that particular sin, so that it is necessary by the punishment of the one man to deter others. Consequently, in the tribunal of Penance, the punishment has to be imposed with due regard to both these things: and so a greater punishment is not always imposed for a greater sin. on the other hand, the punishment of Purgatory is only for the payment of the debt, because there is no longer any possibility of sinning, so that this punishment is meted only according to the measure of sin, with due consideration however for the degree of contrition, and for confession and absolution, since all these lessen the punishment somewhat: wherefore the priest in enjoining satisfaction should bear them in mind.
Reply to Objection 1. In the words quoted two things are mentioned with regard to the sin, viz. "glorification" and "delicacies" or "delectation"; the first of which regards the uplifting of the sinner, whereby he resists God; while the second regards the pleasure of sin: and though sometimes there is less pleasure in a greater sin, yet there is greater uplifting; wherefore the argument does not prove.
Reply to Objection 2. This punishment of seven days did not expiate the punishment due for the sin, so that even if the sinner died after that time, he would be punished in Purgatory: but it was in expiation of the irregularity incurred, from which all the legal sacrifices expiated. Nevertheless, other things being equal, a man sins more grievously under the New Law than under the Old, on account of the more plentiful sanctification received in Baptism, and on account of the more powerful blessings bestowed by God on the human race. This is evident from Hebrews 9: "How much more, do you think, he deserveth worse punishments," etc. And yet it is not universally true that a seven years' penance is exacted for every mortal sin: but it is a kind of general rule applicable to the majority of cases, which must, nevertheless, be disregarded, with due consideration for the various circumstances of sins and penitents.
Reply to Objection 3. A bishop or priest sins with greater danger to others or to himself; wherefore the canons are more anxious to withdraw him from sin, by inflicting a greater punishment, in as much as it is intended as a remedy; although sometimes so great a punishment is not strictly due. Hence he is punished less in Purgatory.
Reply to Objection 4. This punishment refers to the case when this happens against the priest's will: for if he spilled it willingly he would deserve a much heavier punishment.
Source: Summa Theologica (New Advent)