I think handhelds and home consoles should be in categories all by themselves. Heck, VGC seems to think so, too.
Of course this is in no way to try and downgrade the DS's numbers, because that would just be foolish and bias. The DS numbers are incredible. But when you actually take a step back and look at it, it just seems silly to compare the 2. Here's my thoughts anyway.
1. The DS is just way more accessible. You have everything you need when you purchase the console. No TV's, no wires, and no real extra space needed for consoles and controllers. This is probably why the handheld is much more popular in Japan than home consoles, considering their smaller living spaces. And gaming on the go has always had appeal with children of all ages.
2. The price is a major contributor. At launch, the PS2 was $299 ($381 by today's standards). The DS, on the other hand, was half that price at launch, $149 (though less than half by todays standards, $174). Lol, I don't think I need to explain this one further.
3. And another important factor, the gaming market has expanded. The PS2 (as well as the PS1) helped increase the popularity and the audience for gaming. The DS was able to benefit quite a bit from this expanded popularity and audience, as it came out 4 years after the launch of the PS2.
Again, this isn't to say DS's numbrs are crap because it had many things in its favor, because they aren't. Just illustrating it's not exactly an apples to apples comparison.