Hotmail, Yahoo, AOL, and Gmail are probably the biggest targets for spammers, especially those using dictionary attacks. If you are going to send spam you certainly will be including those providers in your target list.
Unfortunately for these providers they cannot implement certain restrictions that smaller email providers or businesses might set up. For example, we run our own email server and reject outright email connections from a number of countries. We have the luxury of not needing to exchange email with someone in Russia or China (for example) which allows us to filter out huge blocks of IP ranges (using the country specific RBL's).
So Hotmail users may see spam in high quantity, but it's likely a very small percentage of that actually was targeted to the user. I did not RTFA, perhaps I should do so now, but it makes sense to me that Hotmail may actually have quite a good anti-spam scorecard.