The largest benefit of a larger auditor, IMO, is the reduction in risk of "collusion" between the auditor and the client company. They also tend to have more personnel-per-client than small firms who are often overbooked.
Keep in mind too, the more clients a large auditor has the higher the chance that ONE of them will "slip by" regardless of how careful the company is. So finding cases where a large auditor failed really means less given the huge number of clients they have.
Perhaps all that is true, but in this space, bigger is bettter. Perception is key, and decent sized companies should have decent sized auditors, especially for Chinese issues where the whole sector is looked at with high suspicion..