When Alma-2 hands over the plates and sacred relics to his son, Helaman, he also gives instructions and advice. One well-known passage in Alma 37 is the one about God using small means to accomplish great things. Later, when specifically talking about the Liahona, he repeats this
41 Nevertheless, because those miracles were worked by small means it did show unto them marvelous works.
I don't know why I haven't noticed before, but Nephi says the same thing in 1 Nephi 16, where Lehi finds the Liahona outside his tent. Talking about it, Nephi says in verse 29
And thus we see that by small means the Lord can bring about great things.
Clearly, Alma is making a reference to Nephi's words on the small plates. Nephi brought up this principle first in connection with Liahona and it seems to have made an impression on Alma who expands on it in his admonitions to his son when passing on the Liahona to him.