[by H. J. Rose in A Handbook of Latin Literature (p300), and J. Wight Duff in A Literary History of Rome from the Origins to the Close of the Golden Age (p480) respectively]
However I recently read an article which suggested another possible interpretation of the phrase. The author points out that “Livy is remarkable for the extreme range of styles which he uses in his narrative to achieve variety”, and that it’s therefore a bit of a stretch to characterise his style as smooth and flowing.
A better way of understanding Quintilian’s phrase (so the author argues) is that Livy’s writing is ‘nutritional’ and ‘appropriate for infants’; that is, it is good for students of rhetoric as it provides them with good models to imitate; it helps them to ‘grow up’ as speakers, just as milk helps babies to grow up and mature. The metaphor of milk in the context of education is found elsewhere in Quintilian, in several first century Greek writers, and even in the bible. It also fits with Quintilian’s own methods of education. As he writes in his Insititutio Oratoria (2.5.19)
“For my part I would have them read the best authors from the very beginning and never leave them, choosing those, however, who are simplest and most intelligible. For instance when prescribing [texts] for boys, I should give Livy the preference over Sallust; for although the latter is the greater historian, one requires to be well advanced in one’s studies to appreciate him properly.”It is good for young boys to read Livy not because he is the best historian, but because his writing will help them to mature in their studies.(Quintilian is silent on what girls should read.)
The article finishes up with this helpful summary:
“I conclude, then, that illa lactea ubertas Livi is an assertion that students at the elementary levels of the oratorical curriculum can obtain from reading Livy the sort of supplementary information and stylistic skills which keep their speeches from being bare bones. They will encounter unusual words, rhythmic prose structures, notable tropes, other ornamental features, and a general fullness of style which are the components of ubertas. And they will encounter them in an author who will prove enjoyable and engaging, and thus suitable to their youth.”
[Lactea Ubertas: What's Milky about Livy? Steve Hays The Classical Journal, Vol. 82, No. 2 (Dec., 1986 - Jan., 1987), pp. 107-116]