Don't have any reference but I'd guess it's the volume of the lung, pressure it can generate, and the capacity of your voice box to resonate. I have a very thin voice as well, if I have to give a talk it's really a problem. There isn't very much one can do about it. Right breathing helps to maximize pressure, but at least I get easily out of breath. If I can get a microphone, I could kiss the organizers of the conference!
Besides this it's of course self-confidence.