New Background in 6e Rulebook

[The bearded one]

Well, the text I plucked it from is about splitting up the legions into chapters and in that context it makes more sense to see the 100.000 to refer to individual legions, as the point of the split and that text was to prevent any 1 person from having such power at his own command. Horus was leading the traitor legions, but it wasnt like he was ever personal commander of all 9. They had their seperate primarchs to who they owed their loyalty. The context of the text really means 100.000 marines as an organisational unit.

[Tastyfish]

Starting at 10K each makes sense, with them then expanding rapidly once their Primarchs are found and new recruitment worlds utilized.

Wasn't recruitment and transformation a lot quicker with the Primarchs as well, so the original method is probably similar to current marine recruitment methods but with access to Primarch geneseed you can clone it much faster and more reliably. It's also slightly more aggressive and doesn't need a host already in a state of hormonal flux and development.