I've been reading Real-Time Collision Detection and for loose octrees it recommends expanding each node's AABB length by a factor of 2, making its expanded volume 8 times as large. I am surprised that it needs to be this large, and there must be quite a lot of overlap and unnecessary child node traversals due to this large size. Within the top few levels of the octree, triangles are generally very small compared to the node's volume so I don't see any problem with making their scaling factor much less than 2, perhaps even 1.1 (And then ramping it back up to 2 after a certain depth.)
This is for an octree holding static background geometry so I'm not concerned with how long building it takes.
Is there a mathematical reason for the radius ratio being two?