You say that in real life we would get it linearly, but really we probably don't. After a certain point, the more complex and larger a system gets, the likelihood that it is inefficient rises, or that the operating cost to maintain that system outweighs whatever it will bring in. Think of it this way, a coal plant will certainly provide more power if you add more turbines, but its more complicated then just adding that in. You have to get the workers, ensure there is space, and ensure the rest of the system operates with more output.
In short, we shouldn't really compare a game to real life, but beyond that there is a fair chance that power mechanics will not stay this way forever.