I think the loot you get depends on how much you actually kill. Lets say you join a battle between dark hunters and a vaegir war party. If you let the vaegir war party kill the dark hunters (lol, like that would ever happen), I think you wouldn't get much loot because you didn't do much killing.
This is just a guess, but the reason I think this, is because the game does a similar thing with prisoners. At the end of a battle, you only have the option to take the prisoners that your men knocked out. I know this because as a noob (im still a noob, but...) I joined in many fights with manhunters and river pirates. It never lets me take any prisoners taht they knocked out, but if I knock out one myself, i can take 1 prisoner at the end, if borcha knocks out two and I knock out one, then at the end of the fight 3 of the prisoners will be mine. I don't see why loot would be any different.