Hi Tuvok,
Thing is, these bigger ads are the ones which are used more these days. In many cases they pay more than the smaller ones, but that wasn't our primary concern.
Our primary concern was that because we had less "fashionable" ad sizes, we got dodgier ads from less well heeled companies.
You might complain about having the bright and big expedia ad - but at least you know it's of quality from a reputable company. In this case, bigger not only proved better, but is often much more reliable.
So that's "why" :)
We need the ads in general to pay the bills.