Objection 1. It would seem that slight contrition does not suffice to blot out great sins. For contrition is the remedy for sin. Now a bodily remedy, that heals a lesser bodily infirmity, does not suffice to heal a greater. Therefore the least contrition does not suffice to blot out very great sins.
Objection 2. Further, it was stated above (Question 3, Article 3) that for greater sins one ought to have greater contrition. Now contrition does not blot out sin, unless it fulfills the requisite conditions. Therefore the least contrition does not blot out all sins.
On the contrary, Every sanctifying grace blots out every mortal sin, because it is incompatible therewith. Now every contrition is quickened by sanctifying grace. Therefore, however slight it be, it blots out all sins.
I answer that, As we have often said (1, 2, ad 1; 3, 1; 4, 1), contrition includes a twofold sorrow. One is in the reason, and is displeasure at the sin committed. This can be so slight as not to suffice for real contrition, e.g. if a sin were less displeasing to a man, than separation from his last end ought to be; just as love can be so slack as not to suffice for real charity. The other sorrow is in the senses, and the slightness of this is no hindrance to real contrition, because it does not, of itself, belong essentially to contrition, but is connected with it accidentally: nor again is it under our control. Accordingly we must say that sorrow, however slight it be, provided it suffice for true contrition, blots out all sin.
Reply to Objection 1. Spiritual remedies derive infinite efficacy from the infinite power which operates in them: wherefore the remedy which suffices for healing a slight sin, suffices also to heal a great sin. This is seen in Baptism which looses great and small: and the same applies to contrition provided it fulfill the necessary conditions.
Reply to Objection 2. It follows of necessity that a man grieves more for a greater sin than for a lesser, according as it is more repugnant to the love which causes his sorrow. But if one has the same degree of sorrow for a greater sin, as another has for a lesser, this would suffice for the remission of the sin.
Source: Summa Theologica (New Advent)