We present a theoretical description of how continuous monomer production affects the focusing of nanocrystal size distributions in solution. We show that sufficiently high monomer production rates can drive a decrease in the polydispersity even as the average nanocrystal size increases. This is in sharp contrast to Ostwald ripening, where polydispersity increases with mean crystal size. We interpret several experimental nanocrystal studies through our model and show how production-controlled growth promises exquisite control over the size and polydispersity of functional nanocrystals.