Herbert wrote Dune with one word in mind: epic. Then he said to himself, "Why stop there? Why not make this beast of a book epically epic?" And so he did.
Everything in Dune happens on a grand scale. In fact, the characters can often get buried beneath all the information the reader has to sift through—information given (let's be real) to ensure that the scope of the story can grow larger, more impressive. Take chapter 6. There, Duke Leto is having a heart-to-heart with his son, but the conversation is less about Paul's sense of displacement from his childhood home than on building Dune's universe. Just have a look:
"Few products escape the CHOAM tough," the Duke said. "Logs, donkeys, horses, cows, lumber, dung, sharks, whale fur—the most prosaic and the most exotic… even our poor pundit rice from Caladan." (6.22)
"Knowing where the trap is—that's the first step in evading it. This is like single combat, Son, only on a larger scale—a feint within a feint within a feint… seemingly without end." (6.32)
"You hear an occasional muttering about the Emperor's training cadres, but the balance of our civilization remains the same: the military forces of the Landsraad Great Houses on one side, the Sardaukar and their supporting levies on the other." (6.48)
See what we mean? Between House Atreides, House Harkonnen, Arrakis, the CHOAM, the Padishah Emperor, the Landsraad, the Sardaukar, and so on, almost no room remains for Leto and Paul to just be, you know, father and son. Epic and grand to a fault.
But this tone conveys itself through more than just the story. Even the bits and pieces surrounding the story help to lift Dune into the realm of the epically epic. Every chapter starts with a quotation from a song or book in Herbert's universe, serving to help the reader feel that a larger universe exists beyond the one we can access directly. Then there are the appendices at the end of most editions, which flesh out the religion, genealogies, characters, languages, and politics of the universe.
Arthur C. Clarke once said he could only compare Dune to The Lord of the Rings in terms of scope (source). Boy, he wasn't joking.