Yes I know Domino's has clipped corners on their box, but it isn't exactly round by any means. Papa Johns attempts to make use of this extra space by tossing in some dipping sauces and a pepper or two (which almost always end up in the trash), and occasionally you will find a pizza place that actually makes square or rectangular shaped pizza, but it is pretty rare.
I wonder how much extra space is delivered to American households in the average day. I'd say the guy who can invent a round pizza box that can be produced economically (at least as cheap as the more common square boxes) is going to be one very rich man.
Maybe they should get one of the Ikea designers to work on this... those guys are experts at efficient design and know how to pack the absolute most amount of product into the smallest possible container.