I don’t think the dungeon was fully grown as a Big D dungeon when the spire popped out of the ground. First of all the dungeon didn’t have a harbinger. We only know that Ricther had one day and night to choose a harbinger after he claimed the dungeon. He only assumed in the book that it might choose its own harbinger if he didn’t. When the spire popped up out of the ground the dungeon couldn’t use any magic because it hadn’t awakened any magic at that point. Nor could the dungeon spawn any creatures or monsters because it had never consumed anything. Upon its arrival this dungeon hadn’t even offered its first quest to its dungeon master. There is no way you could consider Ricthers dungeon a fully grown big”D” Dungeon. When it landed in the Mist Village. It couldn’t even connect to the Labyrinth. If Ricthers dungeon was a full grown dungeon why would it had needed any protection for the first year of its life? If it had been it wouldn’t needed Ricther to defend it twice.