Venting the rear axle

[DBasher]

How much of an axle breather is needed? This picture was taken of Greg's Mustang and I don't understand why such a big can....When I played off-road we'd just run the vent high enough to stay out of the water, no can just a filter and check valve, I do realize it was low rpm compared to a track car.

I'm putting together my 9" and would like to understand this a bit better.

Thanks
Dan