I suspect that the quality of the advice which YC can bring to bear on a given problem has improved more quickly through experience and contacts than the effects of larger class size has degraded the availability of any particular partner.

It would make sense if YC has added partners with strengths which fill the gaps in the existing partners expertise. To put it another way, PG is the public face of YC, but it has been shaped by JL from the beginning, too.

http://ycombinator.com/start.html