Well, do you mean body size or antler rate? Body size, most of your mid-western whitetail are pretty well fed and get up there in weight. Basically, the further North you go, the bigger the body size of deer is the general rule. As far as antler rates, South Texas has produced some absolute monster racks but a whole lot of those monster racks came from ranches that do a whole lot of genetic "culling" and run breeding programs as well as feed programs to "enhance" the deer population. Not to mention those racks "look" a lot bigger on the "large dog" sized bodies
of them little ole southern deer. Rack size is relegated to 3 things, genetics, food quality and quantity, and age. South Western Illinois has produced some monsters both body as well as rack. So has Wisconsin and Kansas.