What's with the beyond-casual introduction of “stone-giants” in The Hobbit?

Question

In Chapter 4, "Over Hill and Under Hill", I suddenly read this:

When he peeped out in the lightning-flashes, he saw that across the valley the stone-giants were out and were hurling rocks at one another for a game, and catching them, and tossing them down into the darkness where they smashed among the trees far below, or splintered into little bits with a bang.

I backtracked several pages in order to attempt to find any mention of "stone-giants" or "giants", but this seems to be the first time they were mentioned.

What "stone-giants"?! Are they giants made out of stone? Like the trolls that turned into stone? Bigger? Moving mountain-giants?

At first, I thought this was a poetic way of describing the mountains themselves and the rocks falling down from them, but the narrator seems to be talking about literal giants made out of stone? Which are just suddenly mentioned without as much as a basic description?

While there is a mention of "various horrors/dangers" in the mountains, I didn't expect them to just have giants made out of stone existing there, without some sort of explanation or history. The sentence reads just as if they had been already established, the way it says "the" stone-giants.

I searched an online copy of the book and there is no mention of the literal word "giant" before that in the same chapter or the previous one.

Answer 1

That’s what you get with the Hobbit, don’t expect to find any sort of additional detail. The Hobbit was in Tolkien’s mind intended for children so things aren’t always explained to the degree they might be in the Lord of the Rings or the Silmarillion.

With most other subjects you might expect to find more in Tolkien’s other writing, but not giants. They don’t exist in any of the final published works, only other references are in early drafts that you have to be very careful with, considering how much changed between them and the final drafts.