The surface water system is puzzling?

This is the tank that Claude is building. The big diameter ribbed pipe exiting on the left is the surface water drainage. The smaller diameter one crossing the tank is the foul water pipe. Now, as I understand what they said, if there is a downpour and the water rises as high as the foul water pipe there will be an opening on the top to allow this excess water to run through the foul water pipe to give extra drainage. I may have missed something  but I can't help but wonder why they didn't put in a bigger diameter surface water pipe as they had the ditch dug. It was big enough!