The topic is how many security vulnerabilities exist in various operating systems, NOT how many of those vulnerabilities have been exploited.
The question you pose is a "theory to practice" one. In theory, they are about equal. In practice however, you need to look at how tight the nut is between the keyboard and the chair.
Linux users are usually more computer savvy, so I think Linux is the most secure operating system. Mac users generally don't know or care how thier computer works, but they are also less likely to do things that make their systems less secure. Many Windows users shouldn't even be allowed to own computers. They are clueless to the point of being dangerous.
Anyway, any OS is as secure as the person who uses it wants it to be. Security isn't easy, convenient or fun. If you want to be secure, you have to do without certain things. Add functionality, take away from security.
In my experience, Windows users are generally more ignorant about their computers than Mac users.
I've also observed that when Windows users switch to Macs, they suddenly find their computer experiences to be more useful and enjoyable.
That is one of the problems that are inherent with that largest market share. That could be argued with any product or industry that owns 90+ % of a given market, but there aren't any others.
One advantage is that the clueless most likely know someone, perhaps a family member or friend, that could hep them out with a problem issue.