By definition, pirated versions are not counted on sales charts, inasmuch as pirated versions have not been sold. That’s why they’re pirated. Again, I ask you, so what? Whether or not your install base figure is accurate has nothing to do with whether or not OS X is somehow as insecure as Windows.
If you were on a deserted island, with only a handful of people alive after a nuclear holocaust, would you feel the need to have an ADT alarm system? Kind of how I look at Mac users not needing to have an anti-virus prog installed.