Dogs are derived from wolves. today, dogs are very diverse, varying in size from very small to very large. what led to this variety?

I would say evolution because when the wolf's became domestic they no longer needed to hunt down their pray because the humans would feed them, and eventually they lost some Instinct on hunting. Humans also took them all over the world and they slowly started to adapt to their surroundings, so the smaller dogs could be small because of evolution but it is more likely to be because of cross breeding and the genetics they got from that.