I don't think our dragon was quite at that level when it was empowered by a weak darkness amulet.
So, a dragon that is empowered by all eight Prime orbs(well, five Prime orbs and three shards that serve as replacements), and the Ultimate Bacon orb(which is supposedly stronger than the one that the SMUDD used), and was stronger than the SMUDD during their fight(if you remember, the main reason we had trouble with the SMUDD was that shadows kept re covering the parts of it that we destroyed until we got rid of the Ultimate orb and let the sun back out), and has really only grown stronger since, is not at the same level as the SMUDD when it got empowered even further, to the point where it had equal destructive capability as caitiff even in their base child form, which before the empowerment, they needed to be a full grown Titan dragon to do? Also, the Doom Amulet was by no means weak. It Titan-ized a Titan. Not even the regular Dragon Amulet can do that.
Though, I will agree that our dragon didn't cover the entire world. But it wasn't due to a lack of power. It was due to a lack of commitment. We even see in one of the black winter cutscenes that our dragon is actually fighting against their orders somewhat, so it's very likely that they only doing the bare minimum amount of darkness covering to please caitiff, or the grounds that it will leave the hero alone.
Caitiff was specifically focused on conquering Lore (where we were), with the intention to spread its reach later on.
Lore is the whole world, though. We are in the kingdom of Greenguard, on the continent of Dragonia, on the planet known as Lore.