User blog comment:Khalael/Canon Policy Discussion 1: Space Marines/@comment-25027290-20131103063448/@comment-2253300-20131103065045

Yes, but even so the reason that the Codex set the limit on numbers is because a Space Marine force the size of a legion could do enormous amounts of damage if they went renegade, as they did during the Horus Heresy. Thus the Inquisition has an interest in making sure that Chapters don't become to large, as they may become a threat to the Imperium. Besides often Users make excessively large Chapters, often with very insufficient reasons for the increased size. This is the reason for the rule.