Of all the different army lists available in OPR’s Grimdark Future, over half are associated with humans. These lists include units that are or once were humans. One third of all the lists are directly associated with the Human Imperium. This strongly suggests that, despite being one of the most recent arrivals in the Sirius Sector, humanity is the most dominant and populous race.
Interestingly, the answer as to why is implied in Rogue Trader — the 1st edition of Warhammer 40K. Therein, the Legion Astartes, more commonly known as Space Marines, are described in this way:
It is the most powerful and most feared fighting arm of the Imperium. Most of its troopers are recruited from feral planets…Because the feral planets are rough, primitive and untamed, their inhabitants make excellent fighting material.
OPR pays homage to this concept with its description of the Battle Brother Detachment army lists:
Battle Brother Detachments are battle companies trained on particularly hostile planets to foster special combat traits and military strategies.
In other words, when humans arrived in the Sirius Sector, they specifically sought out worlds with harsh environments in order to bolster and train the most powerful military arm of the Imperium — the Battle Brothers. This means that there are potentially large sections of Sirius space that had been rejected by other races due to harsh conditions and a lack of natural resources that humanity would have seen as valuable. With more space to expand, humanity quickly became the most populous race in the sector.
This is where the story of Cruthanarc begins. When human explores first arrived, they found a planet of extremes. Orbiting just outside the habitable zone of its star, the world is wracked by freezing temperatures. Underneath its highly saline oceans, however, there is almost constant volcanic activity. While this does bring some warmth, it also makes the thin atmosphere toxic. Surveyors saw it as a perfect breeding ground for Battle Brothers.
Colonists were sent to begin limited terraforming and set up the infrastructure necessary to support and maintain a Battle Brother company. Ever adaptable, the humans on the colony thrived. Once everything was in place, the Imperium began a Battle Brother program on Cruthanarc. This is when things went horribly wrong.
The only native lifeforms on Cruthanarc were bacteria. Given their alien nature, it was quickly determined that they posed no real threat to the human colonists. Once the bio-waste of breeding Battle Brothers entered into the environment, however, the native life quickly began to mutate. The infection first manifested in livestock. Creatures bred to be completely docile started to exhibit violent behaviors. Scientists quickly determined the cause. They also discovered that it was airborne and that anyone who simply breathed the air of Cruthanarc was affected.
Normally, such a situation would have triggered a great purge from the Inquisition; however, cooler heads prevailed. It was demonstrated that the infection could not only survive a purge, it could survive every medical treatment available to the Imperium. Cruthanarc sat along an important trade route, so it was very likely that traders and travelers might unwittingly infect themselves by landing on the surface of a partially terraformed planet. It might even attract humans who operated outside the laws of the Imperium. Such a situation, it was argued, might see the infection spread throughout human space.
Instead, an imperial Naval Depot sits in orbit around Cruthanarc allowing ships to safely dock, refuel, and trade within the system without having to land planet side. It allows the Imperium to closely monitor all traffic in the system and warn those on the surface when their airspace has been violated. No ship is allowed to leave the surface and anyone who lands, wittingly or no, is hunted down by the Cruthanarc military and the Battle Brothers who were originally supposed to use the planet as their home base. These forces shoot first and ask questions later.
In the meantime, medical teams on the planet have successfully found ways to slow the progress of the infection and continue to seek for a cure.