It's really hard to predict what will become a billion dollar company, especially with something as early as Product Hunt. We have several bets to place but what you see today may look a lot different 1, 2, 3 years from now.
Sorry but that kind of logic is a cop out. I can point to tons of other startups attacking small markets that could also become big. The founders are super talented too. I've met many of them personally. If the market you were attacking was expanding at a fast pace then your argument would make sense.
I would love to see a yc partner shed some light here.
Julez Maltz has a good post about the importance of non-consensus opinion in startups: http://techcrunch.com/2011/08/20/don%E2%80%99t-follow-the-cr...