And which Harry Potter did not turn into, however fat the later volumes became.
Or, for that matter, Lord of the Rings, which was longer than originally intended, but still contained.
I notice that Lord of the Rings revolves about the very end of the issues with Sauron. Everything else is relegated to backstory. Furthermore, Tolkien keeps a tight control on point of view: you never get a point of view character introduced except first through the eyes of another (except, of course, for the omniscient narrator in the opening 0:). And not often even then. The book sticks faithfully to the members of the fellowship and the scenes they appear in.
Harry Potter takes a bit longer, but structures its story as a bildungsroman. Which does not marry entirely happily with the epic elements, but does not allow it to sprawl too far, what with neither Ron nor Hermione getting major subplots. At times, the background elements do not so much add to the story as lure attention from it, but she doesn't let it bloat the book too much.
After all, trying to do a major epic of a fantasy war is like trying to do one of a real life war. One can imagine an epic about some Marines in the Pacific War, or some GIs in Europe, but trying to turn in an epic about all of World War II would burst the bounds of any series. You can't do an epic like that. You have to take a slice out of it.
An observation as old as Aristotle, commenting on how Homer didn't try to do the whole Trojan War.