Just because we understand the physics pretty well doesn't mean we've done all the detailed engineering for a production plant. That's the level we're at right now, for tokamaks.

Building extremely large plants right off the bat would be a lot slower and higher risk. I think CFS is planning a plant around 500MW, after doing a smaller prototype that should modestly exceed breakeven.

The CFS reactors are much smaller than ITER because they use stronger magnetic fields, using newer superconductors. Tokamak output scales with the square of size but the fourth power of field strength. But there are limits: how much current the superconductor can support without quenching, and the mechanical limits of the reactor. CFS is nearing those limits, so they'd have to build a much larger reactor for such high-power reactors. It's probably more economical to build lots of smaller, identical reactors.

Regulators are turning out to be pretty rational on all this. The NRC has already put fusion reactors under the same regime as medical devices and particle accelerators, rather than the extremely slow process that fission reactors have to deal with. The UK is doing the same. It makes sense due to fusion's inherent safety and lower proliferation potential.