NPC sizes mean literally nothing.
Not figuratively nothing, literally nothing. Arthas is almost Gronn-sized at several points in Northrend for no real reason.
I could fill a post cap with examples of this. Another is vrykul, which vary between tauren size and twice the height of a tauren ingame with no consistency or explanation.
Do not use NPC scaling as indication of anything, and you may save yourself some headaches.