Macs are more vulnerable to what? Crashes? Lockups? Spyware? Viruses? Please elaborate.
Also the amount of "fixes", as you refer to them, include version upgrades that provide more capabilities, not just security patches. It's like when I upgraded to IE7 - I didn't consider that a "fix", but the 5 patches I had to download for IE7 afterwards were definitely "fixes". And that was for one Microsoft's software products, not hardware designed from people around the world.
You talk in size, which is unimportant, so now compare the quantity of patches in 2006 and 2007 that Apple released compared to Microsoft. I'd rather have 4 updates compared to 30 in a year.
It may be that Microsoft has more patches because hackers hate them and target their product (seems to happen with market dominance), but I'm simply a computer user and enjoying my Mac.