The idea behind splitting the 9th Circuit Court of Appeals is to make the court smaller and thereby increase collegiality. The trouble with that approach is that "more than 50 percent of the Ninth Circuit caseload comes from the Central District [of California]." This is more than the caseload of some other entire U.S. Court of Appeals circuits. California as a whole accounts for about 63% of the 9th Circuit's work, and about 13% of the number of appeals lodged nationwide.
The First Circuit, which includes Maine, New Hampshire, Massachussetts, Rhode Island and Connecticut, is the smallest of the U.S. Court of Appeals Circuits which manages with just six judges, about half as many as are necessary to consider appeals from the Central District of California alone.
Any court that includes California (and no state has ever been forced to endure the heinous mess of having more than one U.S. Court of Appeals, which would probably develop splits of authority on some issues of federal law), will necessarily be huge.