I primarily sail my War Galleon. I have not noticed any reduction or change in plunder rewarded. As someone else mentioned, it appears random.
I have hit the same ship over and over and over and received different amounts of plunder, different level/number of enemies on board, etc. If I have made any observation, it seems to me the harder the ship is to take, the more plunder that appears to be rewarded (similar to reps).
Zeppers lvl40
http://pirates.teamofx.us