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I mean Ultras reached 250k with their 500 worlds. Meanwhile Irok Warriors reached near that with just Olympia at 180k. Apparently their gene-seed was the equivalent of blug-amd-play. And then the Dark Angels were around that ballpark but they got the advantage of being the first and the only one SM legion for a while meaning no competition in recruitment. And then there is Luna Wolves at 170k. And what their factor for getting big was is beyond me. They recruited from one world, didn't enjoy the plug-and-play gene-seed of the IV. nor were they close to being the I. legion.

And had Perturabo and Iron Warriors had Guilliman's territories, it would have been very interesting to see what they'd have accomplished in terms of legion size with 500 worlds rather than the one they got.

It's a mixture of factors that you touched upon already for some, basically it came down to:

- Logistics and administrative capabilities of the legion/primarch. Like you said already, Guilliman's ability to handle and establish supply lines, including recruits, through his 500 worlds meant he had a steady stream of war material and manpower pumping into his legion at a prodigious rate. Similarly, Perturabo had garrisons across the breadth of the Imperium, meaning his legion had access to several world's resources and many were self-sufficient outside of the main legion forces.

- Geneseed stability/compatability. There's a reason why a significant percentage of the current marine chapters in 40k are based on Ultramarine stock. This ensures a higher success rate amongst recruits that get converted into Astartes. As you noted already, Iron Warriors geneseed was another example of high conversion rates for Astartes. This is in contrast to more specialized geneseed like that of the Space Wolves and Thousand Sons, where they typically were forced to recruit from a specific system/homeworld.

- Battlefield specialty/role and casualty rate. These go hand in hand, as legions like World Eaters and Iron Warriors had high attrition rates, the former due to their shock assault role while Iron Warriors used prolonged human-wave siege tactics that focused on grinding the enemy down. There are also cases where exceptional losses caused by major campaigns, like the Dark Angels' xenocide against the Rangdan caused them to suffer heavy losses, leading them to fall behind the Ultramarines despite their initial lead in numbers.

 Tyzarion_Kronius wrote:
I mean Ultras reached 250k with their 500 worlds. Meanwhile Irok Warriors reached near that with just Olympia at 180k. Apparently their gene-seed was the equivalent of blug-amd-play. And then the Dark Angels were around that ballpark but they got the advantage of being the first and the only one SM legion for a while meaning no competition in recruitment. And then there is Luna Wolves at 170k. And what their factor for getting big was is beyond me. They recruited from one world, didn't enjoy the plug-and-play gene-seed of the IV. nor were they close to being the I. legion.

And had Perturabo and Iron Warriors had Guilliman's territories, it would have been very interesting to see what they'd have accomplished in terms of legion size with 500 worlds rather than the one they got.

I suspect that the bottleneck for marine recruitment may have been geneseed.

Another factor was access to the legion primarch.

The crusades used a different method of marine recruitment, one that churned out marines in something like a year from start to finish, but it required physical access to the primarch to "stabilize " the gene seed. Thus, legions that found their primarch early tended to be bigger, which is why the lunar wolves/ sons of horus were so big, because they were the first to use these methods as horus was the first found.

Its implied in some lore that this rapid training system, adopted to keep up with the brutal attrition of the Great Crusade, also had the affect of both increased mental health issues in the legions, abd heavily imprinting the primarch on the marines and helped lead them into rebellion because you just had to win over the primarch and his legion would mostly follow.