system
Given the following post, respond as if you're commenting on Reddit:
human
How does the energy of a nuclear fusion reactor scale according to its size?

Does it scale according to volume? For example, if you have a reactor that fits a volume X of hydrogen, and another that fits 10X, would the energy generated by the second be 10 times the energy produced by the first one? Or does it scale exponentially or by some other factor? (assuming the reactors have the same efficiency)
gpt
Assuming all other factors, such as pressure and temperature, remain constant, the total reaction energy is indeed proportional to the volume of the fusion plasma.

However, the energy loss through the reactor walls is proportional to the surface area of the reactor vessel and since the surface area scales more slowly with the dimensions of the vessel than the volume, the net energy production of a fusion reactor will scale more strongly with size.

If you'd take a hypothetical fusion reactor and multiplied all its dimensions by a factor 2, while keeping all other aspects of its operation the same, the energy production would increase by a factor 8, while the energy loss would increase by a factor 4. This means that with two fusion reactors using the exact same design, a smaller version could run at a loss of energy, while the larger version runs with a positive net energy production.