This law imposes a diseconomy of scale on fusion reactors. All the energy produced in a fusion reactor must go through its walls, so if there is some limit on

This law imposes a diseconomy of scale on fusion reactors. All the energy produced in a fusion reactor must go through its walls, so if there is some limit on the power/area through that wall, the volumetric power density of the reactor will decline as it is made larger. So once a fusion reactor is large enough to achieve adequate Q, it should not be scaled up further. Contrast this with a fission reactor, where the heat transfer limit comes from transfer through the surfaces of the fuel rods. This surface area increases in proportion to the volume of the core, not with the area of the boundary of the core.