I've only seen people paid to work on the biggest projects, often only when it's become entrenched in everybody's workflow (Linux, some GCC developers, some Python devs), had the source code released after closed development (OpenSolaris, Firefox), or got taken over by corporation (Clang and Apple, khtml/webkit/Chromium with Google and Apple).
The vast majority of OSS projects IME are tiny, have few developers, and not many users outside of the developer.