That's 20k BTC that each person is transacting per week, or almost a quarter of a million dollars. Again, this is per person, per week.
Ok, so maybe this is happening more than once a week. In that case, he can get away with charging lower fees. However, he could also get away with having a lower reserve, so that means that far more money than that is going through him each week. You can see how quickly this gets insane
At one point, someone mentioned futures, or some twist of the concept, as a possible explanation. That'd explain the reserve requirement. He might not need to have them on a daily basis, but must have a large quantity on the off chance someone actually wants to take possession of said coins.