I find the Black Plague itself is mostly invention. I think kingdoms of the middle ages always inflated their populations because they had no reliable systems in place for being able to really know. But to purposely inflate your population also immediately makes you a bigger threat and a bigger power (just as China does now).

It wasn’t even until the Renaissance and ‘enlightened’ monarchs who instituted census forms and mailing addresses and formal taxation, etc, that there was a systematic attempt to gauge true population. These monarchs then realized populations weren’t as high as had once been claimed, and to explain away the discrepancy they overstated the role of the plague. That then allowed them to get away with a lot of shady, corrupt stuff. Just as we see now with population discrepancies in an ultra modern world.