While blackfin and skipjack may seemingly fight harder pound for pound than there larger bluefin and yellowfin brethren, remember that if you are comparing pound for pound you are comparing an ADULT blackfin/skipjack vs. a JUVENILE bluefin and yellowfin, hence why they may seem stronger pound for pound IMHO. It's like comparing a full grow man they may weight 150lbs vs a kid who weights 150lbs while they are still growing. Something about being more fully grown and mature that just adds some strength that can't otherwise be matched. I don't beleive it's a simple equation of size of fish is proportional to strength of fight and that you can extropolate what a "100lb blackfin" might fight like. If those fish grew bigger everything would be changed. When you add into the equation the age of fish, recent diet, recent migration distances traveled, oxygen content of water, depth of water, temperature of water, boat anchored/chasing fish, so many variables it's essentially impossible to answer this question. I've caught 50lb yellowfin that fought like wimps and 50lb bluefin that were stubborn for their size but I still wouldn't feel right reaching any conclusion.
Here here, intelligently stated ol-chap. I concur whole heartedly.