PG and crew have been very proud of the way they've been able to scale YC. This seems to be one of the first times where YC has failed to scale?
The key thing about networks is Metcalfe's law: the power of a network is the square of its nodes. This is also what makes the Internet more and more valuable over time.
Those things together mean scale increases value for founders, and what we've learned is those effects are most potent early.
Scaling is an iterative process :)