the first question I have is what rating is your line? second is where where you fishing? my biggest salmon is on the vedder...big ole stinky white all of 42 lbs on 12 lb leader. 30 minute fight on a 10.5 foot rod and a reel that was well maintained (ie smooth drag).
After a long battle like that even you knot quality is critical so there are lots of areas that can lead to disaster on a trophy fish.
Generally speaking though I float fish 8lb when targeting coho and up size to 10lb or 12lb for springs. Not knowing your gear and the area, hard to say why you broke off that big fish. If your drag isnt smooth and a fish runs then that burst may be enough to snap your line.
PS - Fishing spinners and spoons I usually fish 12 or straight 15. unlike float fishing, hardware provokes a reactionary, aggressive strike. in times like that, I dont think leader size is as critical.