I thought I understood how desirability and ranking worked for stake pools, but I was surprised at the turn of the epoch. From reading the design spec for staking https://hydra.iohk.io/build/3156413/download/1/delegation_design_spec.pdf specifically paragraph 5.6.1 (pages 38/39). I came to the conclusion that desirability is a function of:
pledge: higher = better
apparent performance: higher = better
a0: higher is more influence of pledge (same for everyone)
cost: lower = better
margin: lower = better
Nowhere in these functions is the current active stake a factor. And that makes sense because the ranking is supposed to be non-myopic so considering long term rewards, when the pool is saturated.
Currently the ranking seems to place large pools at the top. Making it seem like active stake is indeed a factor. Where did I misinterpreted the design document or has it not been implemented according to (this version of the) design?