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I do indeed have the setting at the default 72 hours (I guess I can go a lot more with the mempool config that I have), but comparing apples to apples my node still had more than double the current 2,200 unconfirmed tx's just a few days ago.
I guess that there really is no "the mempool", even to the extent that some nodes might have mempool going up in size at the same time as others are going down.
are you sure you are relaying everything?
i am no expert in this field but there are restriction that you can put on txs that you accept for example
minRelayTxFee is one of them you may have had it at a high amount, rejecting most of the transactions out there.
https://bitcoin.stackexchange.com/questions/48235/what-is-the-minrelaytxfee