It usually refers to a thick, heavily muscled fighter with powerful hands, good strength, and a stiff chin... I.e. he packs a lot of power, and can take a beating, like a (military) tank...
You'll also hear commentators talk about a guy having a "big tank" or a "pretty small tank", which is reference to their "gas tank". Guys with a "big gas tank" have good conditioning and can fight all day, guys with a "small gas tank" usually gas out quickly, so if they don't knock someone out or submit someone quickly, they lose.