[pfdietz]

You have that backwards.

The bad thing about fusion is that the power has to radiate through the surface of the reactor. That means that when you are at the engineering limit of what that wall can withstand, your volumetric power density decreases as you make the reactor larger.

This is unlike a fission reactor, where heat is transferred through the surfaces of large numbers of thin fuel pins, and where the surface area increases in proportion to the volume of the core.