The Dar Empire was large but it was not larger than the Karridan Empire. It was actually smaller due to the fact it heavily relied on slavery. Their commoners could even own slaves but it also meant their lives were of far more leisure than others.
Slaves could fulfill mundane to necessary tasks and even be part of their army so the vast majority of the citizenry simply made sure to have children and relax through their cultivation.
Why risk their lives in the Hinterlands when a slave could do it? Why spend years collecting herbs, pills, and resources when the slaves could do it? Essentially, why do anything remotely straining when slaves could do it all?
If Lind would sum up the Dar Empire in a single word, it would be hollow.
They had Sun Tiers but he came to learn most of them were pill cultivators or wicked techniques. They did not hone their own strength nor did they really try to do more than the bare minimum.