Objection 1. It would seem that every act increases its habit. For when the cause is increased the effect is increased. Now acts are causes of habits, as stated above (Question 51, Article 2). Therefore a habit increases when its acts are multiplied.
Objection 2. Further, of like things a like judgment should be formed. But all the acts proceeding from one and the same habit are alike (Ethic. ii, 1,2). Therefore if some acts increase a habit, every act should increase it.
Objection 3. Further, like is increased by like. But any act is like the habit whence it proceeds. Therefore every act increases the habit.
On the contrary, Opposite effects do not result from the same cause. But according to Ethic. ii, 2, some acts lessen the habit whence they proceed, for instance if they be done carelessly. Therefore it is not every act that increases a habit.
I answer that, "Like acts cause like habits" (Ethic. ii, 1,2). Now things are like or unlike not only in respect of their qualities being the same or various, but also in respect of the same or a different mode of participation. For it is not only black that is unlike white, but also less white is unlike more white, since there is movement from less white to more white, even as from one opposite to another, as stated in Phys. v, text. 52.
But since use of habits depends on the will, as was shown above (Question 50, Article 5); just as one who has a habit may fail to use it or may act contrary to it; so may he happen to use the habit by performing an act that is not in proportion to the intensity of the habit. Accordingly, if the intensity of the act correspond in proportion to the intensity of the habit, or even surpass it, every such act either increases the habit or disposes to an increase thereof, if we may speak of the increase of habits as we do of the increase of an animal. For not every morsel of food actually increases the animal's size as neither does every drop of water hollow out the stone: but the multiplication of food results at last in an increase of the body. So, too, repeated acts cause a habit to grow. If, however, the act falls short of the intensity of the habit, such an act does not dispose to an increase of that habit, but rather to a lessening thereof. From this it is clear how to solve the objections.
Source: Summa Theologica (New Advent)