Obviously, most of this manipulation falls into the basic accusation and denial categories. The victims will shout huge numbers which emphasize their suffering, while the accused shout lower numbers to emphasize their restraint. Less obvious is the manipulation of the numbers in order to set an example. In fact, just about any atrocity you name can be seen as an example of something normally harmless taken too far. Let's face it.
Big numbers are impressive, and there's a very real human tendency to exaggerate.
Generally speaking, I've seen more numbers shrink than grow under critical scrutiny. The tally for the Spanish Civil War began at a million and dropped to less than half that once someone looked into it. The Persian Gulf War gets less and less bloody with each new study. The riots over India's partition took a million lives in the earliest reports, but now are considered to have cost about a quarter of that. I hate to sound callous, but humans are tougher to kill than they get credit for.