was just a thought though in the IG our hard and fast number systems and designations need not apply.
Just some food for thought along that line:
In general, WH 40k numbers in regard to armed forces are way off compared to our modern day standards.
In WW2 for example, the Red Army (probably closest analogue to the IG in philosophy of warfare) peaked at about 6.5 million men to fight mainly one nation, on mainly one front.
Meanwhile in WH 40k lore, the Angevin crusade conquered the better part of Calixis sector with 17 million troops, divided into 4 battlegroups and a strategic reserve. An optimistic estimate would put each battlegroup at about 4 million troops. So fluff-wise, they took planets with less men than the Russians had in World War II. Even the Ullanor crusade had only 8 million Imperial Guard troops, which seems way too little for an Empire spanning millions of worlds.
Hell, I'm reading Wariors of Ultramar atm, and they're planning to stop a Tyranid Hive Fleet with about a dozen warships, 2 space marine companies, 2 Imperial Guard regiments and some PDF. Unless 'some PDF'=several tens of millions PDF modern day army size logic really need not apply.