Ok, you've inspired me to do some experimentation.
I started a new game, and immediately added a new design with 100 basic reactors and nothing else. Projected ship cost was 12,503, maintenance cost 5,161.
Next, I multiplied the number of each material in a reactor by 10, started another new game with the same settings, and added a new design with 100 reactors and nothing else. Projected ship cost was 125,025, so increased in proportion, but the maintenance cost was 29,271, so increased less than 6 times, rather than the 10x increase I expected.
I'm not sure why the maintenance cost didn't increase as much as expected, but this hopefully bodes well for my plans, as I intend to increase ship construction costs around 4-5x, but maintenance costs by somewhat less (hopefully around 3x), to create a lower ship-count game where each ship really matters and battles are more likely to be between a handful of ships, rather than dozens, and saving ships from a losing battle, or killing most of the attacking fleet, is at least a minor victory.