I don't think it's quadratic for individuals. Individuals give (base + incr * other_pledgers)... the total donated grows "quadratically", but for the pledgers it's just linear growth based on the number of other donators. Since pledges are bounded per-user, it's subquadratic as the project starts hitting the pledge limits of their users. (Which makes sense since of course all the accounts have to balance and users aren't going to be willing to write blank checks.)