It was the fact they carried a world on their backs. They were literally as large as worlds, and when they grew up, this was even more so.
What Leonel could only do through a technique and Incomplete Worlds, the Primordial Terrors could do naturally. Their worlds were their bodies, their bodies were worlds.
This was part of the reason the Void Race was so fearsome as well. After all, their bodies were constructed and Stars, galaxies and nebulae. In fact, Leonel had seen an Ancestor Void Race member before as well, and his size had been enough to swallow up an entire Incomplete World, which was far larger than any Complete World.
But there was only one thing that Leonel could say to this…
They were wholly inferior.
The Void Race swallowed Incomplete Worlds to add to their collection of stars and galaxies, but the Celestial Terras WERE a world.