I also forgot to add the PC users with illegitimate versions of Windows. Worldwide, there may be more hacked/pirated version of Windows running than total Mac systems. I am not stating this as a fact as I do not know, but in third world countries where piracy is rampant you know there is a huge segment running Windows illegally. Millions upon millions.
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.