Depends what you mean by big. I think mines about 80 litres. I did embarrassingly over flow it once on the busy Aire at St Croix lake in Provence. Everyone was sat outside their vans in the lovely warm evening summer sun chatting, drinking and admiring the stunning views over the lake and we were both taking showers. There was a light tap on the door and a Frenchman was stood there pointing at the floor and the river of grey water flowing down the entire aire where everyone was sat.
They were very polite about it and realised it was "douche" water and everyone had a laugh at the silly Rosbif's. I never knew it had an over flow.