Objection 1. It would seem that a man cannot merit an increase of grace or charity. For when anyone receives the reward he merited no other reward is due to him; thus it was said of some (Matthew 6:2): "They have received their reward." Hence, if anyone were to merit the increase of charity or grace, it would follow that, when his grace has been increased, he could not expect any further reward, which is unfitting.
Objection 2. Further, nothing acts beyond its species. But the principle of merit is grace or charity, as was shown above (Question 2, Article 4). Therefore no one can merit greater grace or charity than he has.
Objection 3. Further, what falls under merit a man merits by every act flowing from grace or charity, as by every such act a man merits life everlasting. If, therefore, the increase of grace or charity falls under merit, it would seem that by every act quickened by charity a man would merit an increase of charity. But what a man merits, he infallibly receives from God, unless hindered by subsequent sin; for it is written (2 Timothy 1:12): "I know Whom I have believed, and I am certain that He is able to keep that which I have committed unto Him." Hence it would follow that grace or charity is increased by every meritorious act; and this would seem impossible since at times meritorious acts are not very fervent, and would not suffice for the increase of charity. Therefore the increase of charity does not come under merit.
On the contrary, Augustine says (super Ep. Joan.; cf. Ep. clxxxvi) that "charity merits increase, and being increased merits to be perfected." Hence the increase of grace or charity falls under merit.
I answer that, As stated above (6,7), whatever the motion of grace reaches to, falls under condign merit. Now the motion of a mover extends not merely to the last term of the movement, but to the whole progress of the movement. But the term of the movement of grace is eternal life; and progress in this movement is by the increase of charity or grace according to Proverbs 4:18: "But the path of the just as a shining light, goeth forward and increaseth even to perfect day," which is the day of glory. And thus the increase of grace falls under condign merit.
Reply to Objection 1. Reward is the term of merit. But there is a double term of movement, viz. the last, and the intermediate, which is both beginning and term; and this term is the reward of increase. Now the reward of human favor is as the last end to those who place their end in it; hence such as these receive no other reward.
Reply to Objection 2. The increase of grace is not above the virtuality of the pre-existing grace, although it is above its quantity, even as a tree is not above the virtuality of the seed, although above its quantity.
Reply to Objection 3. By every meritorious act a man merits the increase of grace, equally with the consummation of grace which is eternal life. But just as eternal life is not given at once, but in its own time, so neither is grace increased at once, but in its own time, viz. when a man is sufficiently disposed for the increase of grace.
Source: Summa Theologica (New Advent)