Don't think in terms of level flight. Its true Vmc is not affected by weight in level flight. Vmc is only affected by the weight of an airplane in a bank... in this case 3-5 degrees. Try this....

"When an airplane is banked, a component of the aircraft weight acts along with the horizontal component of lift to create a more effective sideslip towards the operating engine. So for a given bank, the heavier the airplane, the lower the aircraft's Vmc."