My pairing principles

I OFTEN read that pairing best to best is the optimum breeding strategy. The issue I have with this statement is: what determines the best birds? Is it based on show standards and/ or some other criterion such as size? It should be noted that size is not part of the National Bengalese Fanciers Association (NBFA) show standards, although in practice, with all else being nearly equal, large Bengalese will normally beat small Bengalese.

Unlike many other species, Bengalese finches are normally shown as pairs (gender doesn’t matter), with matching being an important consideration. But, as per the NBFA Bengalese standards, type and condition are