[bjterry]

The cargo a plane can carry isn't limited only by the volume of the interior, but also by the amount of weight that can be carried safely in the plane. Airlines try to balance the amount of dense and non-dense (a.k.a. volumetric) cargo to solve for the joint constraint by charging different prices for cargo depending on the greater of its weight cost vs. its volume cost.

Even if it turned out to be practical to double the volume of cargo you could carry, it seems unlikely that it would allow you to double the weight of the cargo, since the engines and the airframe have all been designed around the same set of engineering requirements. The best case scenario would be a decrease in the cost of volumetric cargo, with dense cargo staying the same.

[kelnos]

The article is about having a completely separate plane (glider, really) that is towed by a conventional plane. The glider gets to ride in the powered plane's wake, and presumably the fuel cost to tow the cargo plane is less than the cost of flying two independent planes.

This isn't about making a larger conventional plane that has more volume to put cargo in.

[bjterry]

Right, but if a plane has a maximum takeoff weight of 600k lb, I doubt if it can safely takeoff while being 600k lb AND towing another x00k lb in a glider, which is what would be required to reduce cargo costs by 65%.