I don't think it is accurate to say that there would be as many problems for the Mac as there have been for Win XP and its predecessors if the market share were equal.
Would there be more problems than now? Of course.
Equal? No because the platform is built to be more secure. I recently read an article that had been linked on some blogs screaming about how the Mac was not as safe as touted. The actual article explained three vulnerabilities. One required the user to enter their system password for the nasty program to do anything harmful. One required you to click a pop-up box to permit the dangerous file to run. The other would cause the Safari internet browser to crash if you visited a site featuring an image file with the exploit.
I don't see much useful out of crashing someone's internet browser other than being able to say you did it. I figure anyone who enters their system password to run an unknown program or clicks a box to run an unknown program gets what they deserve.
The OSX operating system is not impervious but it is more difficult to exploit. It's like passwords for your computer or owning a safe. Its not that you can create one that can't be beaten, its how hard do you make it to beat.
blowfish is a weak password
bl0wfish or Blowfish or bl0wf1sh is a bit stronger.
Bl0wf1sh is stronger than those.
Bl0wfishBl0wf!sh is much stronger than any of them.
All can cracked but the last one would require so much effort and computing power that anyone wanting in might not live long enough to crack it.
Right now the Mac market share just doesn't make attacking it worthwhile but even if the market shares were reversed, the difference in strength will still rule out many people doing it.
Imagine three jewlry stores each with a million dollars worth of inventory easily accessible and 100 theives out there interested in getting the loot. The first store has no alarm system and doesn't lock the door at night and leaves the goods out in plain sight. Most or all of the 100 will knock it over. The second has alarms, locks the door and has a second set of bars they lock of the doors and windows. Less will try it and be successful. The third has the same as the second but then takes the goods and places them in an office safe. Even fewer will try and be successful but that won't mean someone won't try and be successful.
That's pretty much the situation with the operating systems. OSX is harder to beat.http://www.microsoft.com/athome/security/privacy/password_checker.mspx