Per Blizzard's own statements, Kul'Tirans are not meant to be a different race. They just created different models for more diversity.
"Non-playable Kul Tiran human models include thin, normal and fat body types. The playable Kul Tiran use the fat body type. They are not intended to be a separate race, they are simply variants on Kul Tirans made because Blizzard wanted bigger and smaller models to fit the various NPC roles and add more flavor to the world. They are not biologically different from other humans.[7][8]"
https://wow.gamepedia.com/Kul_Tiran_(playable)