Genetics and nutrition are only a part of why deer are bigger in some states and it has more to do with the overall health and antler size than average body mass.

Climate is the only reason why, on average, animals get bigger. The farther north you go the bigger the animals get as a general rule. The larger the body the better it is able tomaintain its corebody temperature during the winter months. That's why Florida deer only weigh about 150 pounds, Iowa deer will weigh 250, Canada deer can weigh up to 400 pounds. It's a product of adapting (evolution) to the environment where they live.