"A leading economist answers one of today's trickiest questions: Why do some great ideas make it big while others fail to take off? "Scale" has become a favored buzzword in the startup world. But scale isn't just about accumulating more users or capturingmore market share. It's about whether an idea that takes hold in a small group can do the same in a much larger one-whether you're growing a small business, rolling out a diversity and inclusion program,...