Board Thread:Warhammer 40k General Discussion/@comment-173.238.152.115-20140122073927/@comment-25618639-20150410012911

Well a theory that I know is since that missing primarchs legions were disbanded, their astartes and resources were given over to the other legions. I believe that one legion in particular named the Ultrmarines got a bulk of the space marines from those legions and that is why the Ultramarines were so large during the Great Crusade.