The way I read it, is that drag increases ^2 for length, but because volume increases lift for airships (as opposed to other aircraft) you get this ^3 increase in lift by increasing length. So airship economy increases with size, as opposed to an airplane.

For the square-cube law, a cursory wiki search yields:

> The square–cube law can be stated as follows:

> When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier.

https://en.wikipedia.org/wiki/Square%E2%80%93cube_law