I believed - the last time I read the Catechism cover-to-cover - that upon examining the indices that the only theological writer referenced more than St. Augustine was St. Thomas. So I believed and so I have said now for many years. I chanced to look at it again yesterday and see that my memory deceived me. Perhaps back then I had simply assumed that Thomas would have been, and over time got mixed up about the source of my surprise.
In fact, if anyone really grasped the Western Christian Tradition they would see that it is only appropriate that Augustine should be referenced there more. Frankly, Augustine contributed more to the Western Theological and Philosophical Tradition than did Aquinas. Few who knew what they were talking about would dispute that. Now, does that mean that Augustine wrote a better textbook on Catholic Theology than Aquinas? No, but what's more to the point, Aquinas preserved Augustine's most significant insights - his teachings on the Trinity, on the sacraments, on grace - and put them with a bunch of other essential things - doctrine of the angels, of the hylomorphic nature of man, epistemological realism - to produce the greatest theology book ever: the Summa Theologica. Thomas was as tall as a giant standing on the shoulder of a giant. Thomas was himself not as tall, but still over seven feet.
Now, this is not to say A) that the Summa will itself never be surpassed (funny that after nearly 800 years it hasn't been!) or B) that Augustine did not write the 2nd, 3rd, 4th, and 5th most important theology books at least:
1) Summa Theologica, St. Thomas
2) De Trinitate, St. Augustine
3) City of God, St. Augustine
4) Confessions, St. Augustine
5) On Christian Doctrine, St. Augustine
6) Sentences, Peter Lombard
7 Peri Archon, Origen
8) Exposition of the Orthodox Faith, St. John Damascene
You'll probably think I'm a little biased here. The problem is this, Augustine wrote about a 100 books, and certainly half of them are greater than anything anyone else has ever written on any theological matter. But it is kind of silly saying that a work like Confessions, a protreptic autobiography, is a great theological summa. But its foundational theological ideas are ingenious and incredibly theologically important, one might even say more important that anything Thomas ever came up with. And, so, rounding-up in one case is justified with rounding-down in the case of the other 46 works of Augustine that have not been included in this list, most prominently missing including, Contra Faustum, The Unfinished Work Against Julian, The Literal Interpretation of Genesis, and the Homilies on the Gospel of John.
So in summary, Augustine quoted in Catechism more than Thomas.