I think if you watch the tank levels the bigger black tank is used for printing, however if you print a lot of pictures, you'll notice the smaller tank being used more. Printers use more black than anything.
It makes perfect sense to me. Even though the colors are beautiful and dress up anything you want to print, the black ink is still the workhorse for printing so unless you want to spend all your time changing the cartridges, they installed mostly black ink. And that is okay with me.