My little BPs the same way with food,he will eat rat pups,but it can take him hours on end sometimes,others he won't and others he will strike in seconds. Now put a mouse near him,done deal within seconds he strikes.
my ghost is about 360 grams and can eat a small rat,but I feed him fairly large weanling rats.
my black pastel is about 100 grams and is eating rat pups/small mice.
Feed according to the fattest part of the snakes body..don't stress yourself about weighing the prey,it's just a way to drive your self crazy. Just eyeball,snakes fattest girth and find a rat or mouse accordingly,even a little bigger then the snakes fattest part is fine. It's that simple..simple simple simple,why make things a hassle I just don't get it.