Size is not a part of the value. Upper right it gives a 1-4 value of the fish. I know that 1 = one raw fish. And it goes up from there. Tuna used to always give 4 raw fish but now a level one tuna gives just 1 raw fish, as do all the rest. So your job since you are interested is put a single level 2 fish in your inv and go play with the cauldron and report back.
Thanks in advance.