No, unless SMF does it automatically, but I'm pretty sure that posts last forever.
You're right, the idea of increasing both entropy and data size probably doesn't explain it. But it's the best thing I could think of .... I suppose I can check if the actual uncompressed data is smaller..
And it's true, the uncompressed database dump is actually smaller.
My question is, why? The only databases that might have activity are SMF and Joe, and I doubt Joe's has daily activity. So SMF must be somehow shrinking....