It seemed that in Storm of Iron, this particular war band made a deal with Abaddon to essentially grab that prize and give it to him in return for not having to be in the Black Crusade. The Iron Warriors have their own methods compared to most war bands and legions in creating more warriors for their legions. They are more of collectors than invaders really. Based on most stories they seem to pick targets they can win in order to take resources and slaves.
I think thats the best explanation for their appearance as a larger legion. Word Bearers and Black Legion are obviously bigger because they have bigger ambitions.
I think thats the best explanation for their appearance as a larger legion. Word Bearers and Black Legion are obviously bigger because they have bigger ambitions.