One problem with epic fantasy (or SF) is that, unlike an epic tale of historical fiction, you can't do against the backdrop of a known epic event. An epic tale of WWII would probably concentrate on as few characters as the main ones of The Lord of the Rings, but taking the background more of less as read.
Harry Potter has perhaps more of a disadvantage, because it's a bildungsroman as well. Keeping it all in Harry's view kept a lot of things off-stage. Take the Tournament, which is, on the face of it, not consistent with Dumbledore's character, even as fully revealed in the end. To foment unity in the wizarding community, he would have come up with something less lethal and less focused. A longer work might have scenes where he was forced to take it as a price of getting anything, or even continuing in his position. He would have put a good face on it for the students, so it would all be invisible to them.
Of course, it might have turned into an interminable series, then.