Reactors take fuel and produce energy. Once the energy has been produced, it would take more energy than you get reusable energy out to convert it to its initial state (some other reactor might be able to use its waste products, however). Except in the event that it has a vastly lower consumption than a larger reactor--which would make it produce orders of magnitude less, comparatively, and generally less efficient--a smaller fuel source will run out faster than a larger one.

If it doesn't need refuelling, what you've created is a perpetual motion machine, which is impossible. :P