I have seen people say pressure cooking and tin canning only softens bones in small fish making them edible, however I also read it works for bigger fish. Do you know which is correct and why?
The bigger the bone, the less you will be able to soften the bone. You can increase the temperature at which you pressure cook or can the fish, and that will allow for more softening on larger bones. Your results will depend on the tools you use, hot hot you can get them, etc. Give it a try and you'll figure out how big of bones you can soften.