a google search would of probably given you a much quicker answer - usually the case with questions that can be found in normal textbooks.
Vmca is lower with higher weight because (1) heavier aircraft are more stable, higher momentum resisting yaw/rolling, and (2) heavier aircraft produce more lift on the wings, which means when you bank into the 'good' engine you have a greater component of lift acting against the yawing/rolling force, which reduces the requirement of having to use rudder.