That is a self-extracting arm. The outer hex is the cap which is threaded into the arm. When you loosen the inner hex, it pushes against the cap to drive the arm off. You're breaking the bond between the steel crank spindle and the aluminum arm (which is in essence stretched over the spindle) so it does require an initial high amount of torque, after which is becomes easier to turn once the bond is broken.
Just curious, what is the reason why you need to remove the crank arms?