So that is what the Sparrow's Premix Fee Rate is supposed to do, right? Things that make the postmix-ed UTXOs able to be remixed repeatedly and the coinjoin that has more than 5-input-5-output is come from that premix fee, if the current pool participant/condition has a low fee that would reduce the number of inputs and postmix remix rounds availability. Am I understand it correctly?
Each Whirlpool coinjoin has two inputs which are brand new entries to the pool, and these are the two that pay that transaction fee for the coinjoin. In a 5-input-5-output coinjoin, then 3 UTXOs will be free remixes. In an 8-and-8 coinjoin, then it will be 6 UTXOs which will be free remixes. So yeah, if there is a majority of 5-and-5 coinjoins, then the relative speed of free remixes will be slower than if there was a majority of 8-and-8, for example.
Having said that, I don't know if it is these new larger coinjoins, if it is more and more people using Whirlpool, if it is just my own bias, or some combination of all of these, but I am getting more free remixes more quickly these days than I did, say, a year ago.