>>96761785
That's fair, and almost certainly the meta reason. But if I'm gonna make an in-lore reason for it, we do know that the Tau area of space is unusually dense in habitable planets, and they know how to create continent-sized space habitats for their Air Caste, and they know how to terraform worlds to become more livable, and they have ecumenopolises with populations in the trillions.
All those things combined would suggest an empire that, while obviously very much lacking in the sheer expanse of territory the Imperium has, is nonetheless able to call upon way more resources and soldiers than you would expect such a tiny area to have.
