Damn it, azapa!
You lost a nice fish and it probably died for nothing
I doubt the flopper was too small to hold the fish (where did you hit it?). 9/10 times in such instances the flopper was not adjusted properly. It should move freely until an angle of approx. 45° and should firmly lock thereafter.
Simple test:
(1) Close the (adjusted) flopper.
(2) Hold the shaft horizontally.
(3) Hit the shaft in the middle with your palm or fist.
=> The vibration should open the flopper and it should stay open even if you hit the shaft again.
I suggest you read PVE's tutorial again:
Document sans titre
Document sans titre
and readjust your floppers.
Another critical aspect related to big & fighting fish:
=> Always try to keep traction on the shaft / shaft line after the spear has passed through the fish so that the open flopper stays in constant contact with the fish and cannot accidentally close.
Cheers, ulysses