My Bav 30 has 7mm wire shrouds holding up a fractional rig. I'm in the process of replacing the standing rigging and have found that the forestay wire inside the roller furling system is only 6mm. The diameter is consistent with the specification of the stay's connection plate and O-fitting used at the top of the mast.
Is there a reason why the wire diameter is only 6mm despite carrying the forces of the sail and furler at the same time as resisting the counter-force of the swept spreaders?
Yup, angles. The forestay has a much better angle on the mast than the shrouds.