Check out the graph on the left. The curves represent different ideas and different starting points. If you start with 10,000 fans and have an idea that on average nets .8 new people per generation, that means that 10,000 people will pass it on to 8000 people, and then 6400 people, etc. That’s yellow on the graph. Pretty soon, it dies out.
On the other hand, if you start with 100 people (99% less!) and the idea is twice as good (1.5 net passalong) it doesn’t take long before you overtake the other plan. (the green). That’s not even including the compounding of new people getting you people.
But wait! If your idea is just a little more viral, a 1.7 passalong, wow, huge results. Infinity, here we come. That’s the purple (of course.)