alt Hacker NewsValve is actually extremely small, I've heard estimates at around 350-400 people.
They're also a flat organization, with all the good and bad that brings, so scaling with contractors is easier than bringing on employees that might want to work on something else instead.
300 people isn’t “extremely small” for a company. I don’t work with/for companies over 100 people, for example, and those are already quite big.