"[The Reverend Mother said,] 'You, Paul Atreides, descendant of kings, son of a Duke, you must learn to rule. It's something none of your ancestors learned.'" Paul opened his eyes, said: "That made me angry and I said my father rules an entire planet. And she said, 'He's losing it.' And I said my father was getting a richer planet. And she said, 'He'll lose that one, too.'[…]" (4.47)
Although Leto receives a richer planet, he will lose it. Why? Because he can't harness the full power of Arrakis, so he'll actually be getting less power in the exchange. It's like these people have a 1080p TV but don't have the cables to actually get 1080p on the screen. They just aren't getting their money's worth.