Objection 1. It would seem that expansion is not an effect of pleasure. For expansion seems to pertain more to love, according to the Apostle (2 Corinthians 6:11): "Our heart is enlarged." Wherefore it is written (Psalm 118:96) concerning the precept of charity: "Thy commandment is exceeding broad." But pleasure is a distinct passion from love. Therefore expansion is not an effect of pleasure.
Objection 2. Further, when a thing expands it is enabled to receive more. But receiving pertains to desire, which is for something not yet possessed. Therefore expansion seems to belong to desire rather than to pleasure.
Objection 3. Further, contraction is contrary to expansion. But contraction seems to belong to pleasure, for the hand closes on that which we wish to grasp firmly: and such is the affection of appetite in regard to that which pleases it. Therefore expansion does not pertain to pleasure.
On the contrary, In order to express joy, it is written (Isaiah 60:5): "Thou shall see and abound, thy heart shall wonder and be enlarged." Moreover pleasure is called by the name of "laetitia" as being derived from "dilatatio" [expansion], as stated above (31, 3, ad 3).
I answer that, Breadth [latitudo] is a dimension of bodily magnitude: hence it is not applied to the emotions of the soul, save metaphorically. Now expansion denotes a kind of movement towards breadth; and it belongs to pleasure in respect of the two things requisite for pleasure. One of these is on the part of the apprehensive power, which is cognizant of the conjunction with some suitable good. As a result of this apprehension, man perceives that he has attained a certain perfection, which is a magnitude of the spiritual order: and in this respect man's mind is said to be magnified or expanded by pleasure. The other requisite for pleasure is on the part of the appetitive power, which acquiesces in the pleasurable object, and rests therein, offering, as it were, to enfold it within itself. And thus man's affection is expanded by pleasure, as though it surrendered itself to hold within itself the object of its pleasure.
Reply to Objection 1. In metaphorical expressions nothing hinders one and the same thing from being attributed to different things according to different likenesses. And in this way expansion pertains to love by reason of a certain spreading out, in so far as the affection of the lover spreads out to others, so as to care, not only for his own interests, but also for what concerns others. On the other hand expansion pertains to pleasure, in so far as a thing becomes more ample in itself so as to become more capacious.
Reply to Objection 2. Desire includes a certain expansion arising from the imagination of the thing desired; but this expansion increases at the presence of the pleasurable object: because the mind surrenders itself more to that object when it is already taking pleasure in it, than when it desires it before possessing it; since pleasure is the end of desire.
Reply to Objection 3. He that takes pleasure in a thing holds it fast, by clinging to it with all his might: but he opens his heart to it that he may enjoy it perfectly.
Source: Summa Theologica (New Advent)