Bergmann's Rule is the reason northern deer are bigger.

I think age is the #1 reason for the big deer we see around our parts. In those vast areas, where few humans go and there are no roads, deer get very old. Most of our property is overgrown clear cut. That lets deer have all the food and cover they want in the same place. It does it make hard to follow the hunting magazines' advice: get between feeding and bedding areas.