IMO the size difference is just a coincidence in this case. Tangs aren't hermaphroditic like anthias, wrasses, clowns, angels, etc. are, but gonochoristic like butterflies are known to be. The even trickier part is that tangs don't actually form pairs in the way many take the word to mean, spawning is a mass event for many species.
Anyways, very interesting to see. That tank has to be huge!