Perhaps because Aquinas uses contracted twice in the same passage further compounds the problem. Your reading of the meaning of “contract” as “change” as in reduce in number, size, etc. is not the only one possible in both instances.
Instance 1
In the first use of “contracted,” I would argue that Aquinas means in the sense of limiting because he is speaking of God’s “fullness of being” which leads into the use of contracted. Also, he is arguing that God’s existence cannot be “put into” any created being precisely because that would “limit” existence to the nature of that being. He adds “…because thus it would be limited to that nature.” This is pretty clear.
Instance 2
This seems, on first reading, to be simply a repetition of the same use of contracted as in Instance 1, but that is not necessarily so. He prefaces this use with two important observations:
- “…it is impossible to have a self-subsisting existence unless there is but one.”
- “Accordingly, every thing which exists after the first being, because it is not its own existence has an existence that is received in something.”
Those two phrases indicate that perhaps Aquinas had a different meaning of the term “contracted” in mind here. In this case it could be that he meant “contracted” in the sense of “mediated” or “the means by which something is transferred or conferred.” In this sense a disease might be “contracted” or transmitted from one entity to another.
If the entire sentence is read with this meaning of “contracted” in mind, the sense of what Aquinas writes is completely changed.
Accordingly, every thing which exists after the first being, because it is not its own existence, has an existence that is received in something through which the existence is itself contracted; and thus in any created object the nature of the thing which participates in existence is one thing, and the participated existence itself is another…
Thus, existence of created things is received through “something” *, which distinguishes existence itself (God) from the mediated or participated existence (subsistent ens) of the created object. This contracted “something” through which participation in existence occurs is the degree of separation that does not “contract” or limit existence itself to the nature of the thing created.
Thus the difficulty exposed by Aquinas in Instance 1, is resolved by him in Instance 2. Existence is mediated or contracted to the creature by the nature or essence of the thing (form) which allows participation (as caused effect) in existence without endowing existence itself on the being, which would be problematic, according to Aquinas, because in that case, existence would be “limited” by the nature of each created thing, which is not only impossible but contradicts the ipsum esse subsistens of God.
If each being had actual existence, which belongs properly only to God, then the identity of existence and nature in God would be compromised, since existence would be limited by the nature of each act of created “existence.” Existence would, then, not be simple, but multiplied and complex.
In this sense, I would argue, that created beings “subsist” or, in a manner of speaking, “contract” their being (subsist) through their created “act of being” mediated in the essential nature of each created thing. They participate in existence as “acts” of existence, but cannot have existence as distinct or separated from God’s existence.*
Yes, he is using " contracted " in the sense of limiting, limited by, bound by, attached to, circumscribed by. Pretty darn clear. Also note that created beings can " subsist, " they just aren’t self-substent. Rather their subsistence is a dependent subsistence.