What do Reformed theologians mean that Catholics have the "more metaphysical conception" of God's grace?

From Catholic material to hand, it certainly seems that there is a large metaphysical component to writings about grace. Certainly far more than I've read in Reformed literature (not that I'm very well read in either!) However, as the OP comments that, "The answer may want to highlight the difference between medieval Catholicism vs. 21st century Catholicism ", there is an article that does that. I have no idea if it is on-line or not (it was published in a book in 1975), so there's nothing else for it but to extract pertinent quotes to put in this answer. As there is far too much to give the whole article, I will leave out the theoretical philosophy (metaphysics) and just give relevant parts of the history of Catholic developments. Twelve pages in comes the heading III. Structure of De Gratia and halfway through that is the subheading 2. Brief history of the theology of grace. I miss out the early history to just deal with the OP's request:

d) The Western doctrine of grace was less interested in intellectual divinization and its cosmic aspects and more moralistic in tendency. It was also orientated towards the history of salvation and of the individual by the struggle against Pelagianism. Grace is the unmeritable strength of love for God which by free predestination delivers some men in original sin from the massa damnata of mankind and from their own egoism, liberates their freedom enslaved to sin and so makes them capable of the faith which operates in love (Augustine). In his theoretical works of controversy, Augustine no longer recognizes an infralapsarian universal salvific will of God. On the other hand he is the great teacher of the Church on original sin, the gratuitousness of grace and of predestination to beatitude, and on the psychology of grace.

e) The later patristic period (while preserving the genuine substance of the doctrine of grace of Augustine and of the Council of Orange: D 178-200a) and the early Middle Ages overcame, in opposition to predestinarianism, the thesis of a merely limited salvific will of God which would positively exclude many from salvation prior to their guilt (D 160a; 300; 316-25). The great age of Scholasticism gave precise formulation by means of a new philosophical (Aristotelian) terminology (habitus, disposition, accidents) to the nature of justifying grace, to the process of justification and to the theological virtues. The concept of the strictly supernatural character of salvific grace was slowly elaborated. It was not merely gratuitousness in regard to the sinner.

f) As against the theology of the Reformation, of Baianism and Jansenism, it was necessary to defend (especially at the Council of Trent, sessions V and VI) the freedom of man under grace, the truly inward new creation of man by habitual grace, its strictly supernatural character (in the post-Tridentine period against Baius) and the universality of God's will to grace (against Calvin and Jansenius). The controversy "De Auxiliis", concerning the more precise theories of how to reconcile human freedom with the divinely efficacious power of grace (Molina, Banez) was left undecided in 1607 (D 1090, 1097) and has remained so until this day. Another equally open question is the problem which has been discussed since Petau (d. 1652), under the renewed influence of the Greek Fathers, whether by sanctifying grace a special relation not simply by appropriation is set up to each of the three divine persons. At the present day theology is concerning itself with the use of personalist concepts in the doctrine of grace, with the unity of nature and grace, without prejudice to their distinction, with a better understanding of the biblical teaching on grace and of the theology of the Reformers." Karl Rahner article pp. 597-8, in Encyclopedia of Theology, Burns & Oates, 1981 impression (bold italics mine)

To cut to the chase with this question, the bold italics I have inserted into the quote show the areas of disagreement, and agreement, between the Catholic view and the Reformers view. The quote also shows (briefly) the medieval and the current state of development of the Catholic view. There is no clash with trinitarian theology here. The clash seems to be with predestination (in relation to God's grace).

I hope this will begin to deal with the OPs request for "a more fleshed out comparison between the Reformed vs. Catholic conception of grace". It certainly hints at the metaphysical Catholic concept, which does seem to be more caught up in metaphysics than the stance of the Reformers, which seems to be more biblically influenced (though there is not room here to delve into that). Certainly the Reformers appear to be more in sympathy with Augustine's view where he "no longer recognizes an infralapsarian universal salvific will of God."