To answer your question, I think it is important to understand what the foundation of your new world is. What @Chasly said about rocks is fun and all, but this can only be believable in a world entirely different than our own. As an example, you could change the fundamental laws of the universe (electromagnetism, strong and weak nuclear forces, and gravity). Perhaps that would be too complex, but there needs to be a reasonable way that consumption of rocks could provide an adequate amount of energy, especially considering the fact that rocks don't contain a lot of readily accessible energy for other organic forms of life. Any way of harvesting that energy would likely release too much energy as a result.
Is it important to you that the dragon eats something small? Is it important to you that you completely remove the normal circle of life that we are accustomed to? Can we not have a larger planet? This would make it easier to stay consistent with the rest of your world because you could take what we already know and just make the potential for everything to grow bigger because there are more resources. That may also necessitate that your characters (if any) would also be bigger, and be in a world with everything relatively bigger but not bigger relative to themselves. That may take away the appeal of larger creatures because their power is no longer relevant in a world of relatively more powerful creatures (everything is relative). Maybe in order to maintain the majesty of large dragons you make it a world that visitors travel to, perhaps on accident. Then you could have this world of large creatures that remains impressive (this is sounding a little like King Kong, which may be a good thing?).
No matter what you choose, there needs to be consistency. If what the dragons consume is important, maybe build a new world around that. If something else is more important, build your world around that. State the assumptions of your world, and then we can productively and effectively design a solution that works in your world. If there is no consistency, nor struggle in the world or in its design, then it becomes boring. Ultimate power is not exciting (just ask Superman, who seemed to intentionally develop an allergy to a shiny green rock).
I have many more thoughts on this, if you would like to discuss please contact me directly. My name is Alex Vance, I have no life. All I do is become more educated in our world and that of fantasy worlds in order to form a more perfect union. Vote for me, I approve this message.