I once read that the same species of animals tend to be larger the further from the equator you get. I believe this is referred to as Bergmans Rule. I don't know if Nebraska and Pa. are that much different in distance from the equator. The rule has to do with more mass to withstand the colder weather. Maybe in some points of equal distance from the equator, the animals tend to evolve a little bigger if that area has colder climates over an extended time period.
Just my 2 cents
Dan