>> I don't see how this could grow 100x.

This only means it's not large enough to be investible, that's not necessarily a problem and doesn't mean it's an inherently "bad" idea.

Small ideas can net the individuals that work on them just as much as working on large ideas... the workforce might be smaller but once you divide it up it's all the same. If the idea provides value to the world, you like it, and can grow it without investment, go for it.