This was very surprising to me as well. Assuming that IDs are 8 bytes (they appear to be) and that the mean post is 250 bytes (which seems conservative to me)

This was very surprising to me as well. Assuming that IDs are 8 bytes (they appear to be) and that the mean post is 250 bytes (which seems conservative to me) then the mean fanout is something like 1600. That's big.