I believe it varies, I also think it may depend partially on how hard it was to defeat the ship your plundering.
Seems to me even if I sink big ships... if they were easily sunk (as in previously damaged) I get lower rewards.
I had a tough go of a Spanish Blood Scourge the other day... got stuck as only remaining crewman on deck (with my lvl33 pirate). After surviving and wiping out about 5 waves of Captains, piratas, and conquistadors... I finally won. Pulled in a couple royal chests, a couple treasure chests, and some cargo to boot... was worth a bundle
