"An odd pair of second raters."

Now that I've worked my way through Brahms, I've fallen back on Bruckner again. An odd pair of second raters. The one was "in the casting ladle" too long, the other not long enough. Now I stick to Beethoven. There are only he and Richard--and after them, nobody.

[From a letter Gustav Mahler wrote to Alma Mahler possibly in 1904 (presumably in July), as found in Irving Kolodin's The Composer as Listener]

The reference to the "casting ladle" is a reference to Act five, scene four of Peer Gynt. The "Richard" Mahler refers to is, of course, Wagner, and not Strauss. So who did Mahler think was in too long, and who not long enough, and why?