Most of the weight is in the high number of bearings (the hub shell ones are large too) required for a freehub. The freehub body is pretty hefty too (being steel).
You've probably got over 100g of bearings in the Pro2, whereas a fixed rear hub will only be about 40g.
Then you have the huge bolts...
In all honesty I wouldn't bother trying to save weight on the hub, you will notice it a lot more in rotational weight (lighter rim tape, alloy nipples, etc).
If you want to shed static weight then the best way would be front freewheel.