Ok I'm no expert but this is the way it was explained to me by abiologist friend of mine .
Its like whitetail deer the deer in Canada weight more then the deer in Florida on average takes a bigger hardier deer to live in the the far north regions of their range . Same with wolves.
Ok here is the subspecies differences.
Great Plains Wolf (Canis lupus nubilus)
Also called Timber Wolf and Buffalo Wolf, this is the was most common subspecies of grey wolf in the continental US.
The range of these animals used to cover the whole of the US and southern Canada. However relentless hunting and habitat destruction has resulted in their protection as an endangered species.
Great Plains Wolf has made a comeback and their numbers are rising again.
Gray Wolf (Canis lupus)
The wolf from which most others arise, the gray wolf is the largest of the canid species. They can be found in a wide variety of habitats throughout most of North America.
These animals survived the ice age and are thought to be the ancestor of domestic dog.