the 189 is a modern helicopter and will therefore have a high disc loading (AUM/Disc area) so it will have a fierce downwash regardless of being lighter than the S92. As engines have become more powerful it has allowed designers to reduce the size of the rotors and push air quicker through the fan - that equals faster downwash.
S92 disc loading = 51.91 kg/m2
AW189 disc loading = 49.58 kg/m2