This bug was first reported in 2014 and remains unresolved in 2016. In its present state, the game pushes a "You have fought and defeated the Bigger Fish!" notification for regular "Big Fish"
Are you sure it is not due to inflation? When I kill bigger fish I get a push notification saying "You have fought and defeated the Bigger Bigger Fish!"
This often occurs with me, I just don't bother to point it out. But of course, it needs to be fixed soon. Sometimes confuses me as to what I really attacked!
I highly recommend that you verify the push notification. You may in fact be getting the "You have fought and defeated the Biggest Bigger Big Fish" notification. Unfortunately, the push notification still doesn't differentiate between Eastern Biggest Bigger Big Fish and Western Biggest Bigger Big Fish. I can't wait until the Norther Biggest Bigger Big Fish and the Southern Biggest Bigger Big Fish are made available, as rumor has it, these latter two Biggest Bigger Big Fish have the biggest bigger big BP drops.