By going from a 74mm to a 61mm pulley (for example) you increase the load required by the supercharger from 38kW to 62kW or +60%.
see pic showing the 2 routes....black is stock...green is with "BWK" Belt wrap kit.

You can see if you didnt employ the BWK - you'd have <180deg lap (stock) and lose contact area, but the charger is "requesting more power" (or providing a greater resistance) and so you have a greater propensity to slip
With the BWK discussed here, you increase this to >210deg (give or take) and get better contact area.

Chris - have you (or others) looked into increasing the tension any - say +10 or 15%.
Do we have any tension figures floating around out there?