This one I agree with Arzgoth. Who knows where the Zerg still left behind stuff? They come from another galaxy, and infested a lot on the way here. Maybe Zerus itself is also still covered in a hive cluster
...Kinda reminds me of Tyranids. All the Hive Fleets are so numerous it blows the minds of all the other races. But that's like. Not even their full population.
same goes for every race in WH40K though, as far as I understand. There are a thousand space marine armies, which is only the elite for an example