I believe it is a little of all of the above. The different subspecies of deer may play some role in it, but I think nutrition plays a big part in it. The northern deer also have to carry more mass to get through the winters. Age structure plays a big key. States with better deer management and hunters who are willing to let young deer walk equal older more mature bucks. I think pressure also play a part. Hunting in the Midwest for the first time this year, and seeing unpressured deer in the woods, was an eye opening experience. The deer were not as jumpy, they would enter very large green fields with out hesitation. Several times I had deer come in on me down wind and they would seldom even checked the winds. I think this calmness also helps in there feeding habits, and there weight retention.
