The BR series coilovers use a large 46mm diameter piston in a 53mm diameter damper body to maximise oil capacity.
As the damper unit operates the oil contained within heats up. The viscosity of oil changes as its temperature varies which can lead to changes in the damping characteristic of the unit.
BC Racing only use a very high quality oil with their coilovers but go that extra mile to maximise performance and build their BR range with a large 53mm diameter damper body. This increases the capacity of the unit which increases the volume of oil contained which in turn reduces the peak temperature and so significantly improves damper performance.