Origin of “long pig”, copied from this Reddit comment:

I think you might be right. In A St. Johnston’s Camping among Cannibals (which the OED quotes in its etymology of the term), he describes how:

The expression “long pig” is not a joke, nor a phrase invented by Europeans, but one frequently used by the Fijians, who looked upon a corpse as ordinary butcher’s meat, and call a human body puaka balava, " long pig," in contradistinction to puaka dina, or " real pig."

Which makes it sound like they were just distinguishing between the length of pigs and people.