As the title said. I believe smart matchmaking was introduced in TBC (might be wrong). So if you queue as a 10 man premade in WSG the matchmaker would try to search for another 10 man premade. Same goes for all sizes, if you queue as a 5 man party and you get 5 randoms in your team the opposing team would also be a 5 man party and 5 randoms or close to it.
This is something Blizz should seriously consider doing. Many people will not enjoy the premade fiesta that will happen. And if you are playing alone or just with one or two friends you will have a bad time.