Huh? Can you explain that further? The only way I can think of that those "0-chamber" reactors can be more efficient at producing plutonium is in terms of space - they have 3 columns in one block space, as opposed to 1 extra column per block for added chambers.
That was the impression I got: the economy of scale in favor of raw plutonium output. I'm not personally convinced either that a bunch of smaller reactors will produce more plutonium than 6-chamber reactors will for the same amount of time, but that was the impression I got from his argument.