It's not weight specifically, it is disc loading that determines your downwash speed and hence the RoD required to encounter VRS.
As it happens the light pistons have relatively low powered engines and larger rotors so the disc loading is lower.
It seems that your downwash speed in the hover in feet per min is roughly equivalent to your RoD in autorotation so, once you know this you can avoid VRS by keeping RoD at slow speed to less than half of that figure.