There are more internal conflicts in circuit law. These must be resolved by more en banc hearings of the full court. If the en banc court consists, for example, of 20 judges as opposed to 12, it takes twice the time even to get the decision out. The second cost is that of litigiousness. With a smaller court of appeals, the possible panel combinations of three judges are less numerous and the law is more coherent.
Legal principles are discernible and judicial outcomes are predictable.
As a court grows, so do the possible panel combinations, and the law becomes fuzzier and less distinct. Litigation takes on the properties of a game of chance and litigants are encouraged to come to court for the roll of the dice. When legal outcomes are uncertain, cases are brought for their settlement value and parties lack clear guideposts for their conduct out of court. Judge Wilkinson has set forth the concerns in a more articulate manner than I have. They are particularly applicable to the Ninth Circuit Court of Appeals situation.