Weight classically makes Vmca worse as it reduces, due to the 5 degree bank limit. The lighter the aircraft, the less sideslip is a result of a 5 deg bank and hence the less restoring yawing moment can be generated to help the rudder.
Weight is nominally neutral for Vmcg.
See also here for further discussion...............
http://www.pprune.org/forums/showthr...ht+affect+vmcg