I am not a Pof F expert but I seem to remember being on a course a long long time ago when the tutor who did know about it tried to explain to us thickos why Vmca increases with reducing a/c weight (mass) rather than the other way round as one might at first think.
Draw an a/c flying away from you, imagine it has an engine out. It therefore has 5deg of bank into the live engine. Lift has to oppose weight (lets conveniently ignore any thrust component that opposes weight), but lift has both vertical and a 'sideways' (anti-yaw) components due to bank. If weight is reduced, so is lift and with it its 'anti yaw' component. Thus, Vmca is increased.
Perhaps the clever chaps out there could explain it better than I can - I may even be living up to my name here! If so I apologise - I'm just a pilot.
BS