There has been some argument about what "Qunari" actually are, rather specifically what the word means. Qunari is sometimes used to refer to all people who follow the philosophies of the Qun, but it is typically used to refer specifically to the Ox-men, those with the metallic skin tones, huge size, and horns.
There are also special terms to refer to the culture that predates the Qun, and for converts to the Qun. Its been commented that the Kossith, the culture that predates the Qun, may have had people that hardly resembles the modern Qunari as well.
I think I have it worked out, it has been said in a few sources that the Qunari practice a form of Eugenics and/or selective breeding for the traits they desire.
The term "Qunari" seems to specifically mean "people of the Qun", perhaps this should be taken a little more literally than people tend to think of it. Thatthe "Qunari" are the literal result of the ideologies of the Qun, that this modern race of large horned men could literally not exist without the Qun having created them.
"Qunari" might more accurately mean "People created by the Qun", the selective breeding and eugenics programs of the followers of the Qun putting all converts into a system where their descendents will eventually become horned giants with metallic skin. This may very well account for the diversity seen amongst the Qunari race, as they are in a sense a manufactured race.