The exceptions have nothing to do with tonnage and everything to do with maneuverability, either due to the nature of work or the need to stay in a channel to p

The exceptions have nothing to do with tonnage and everything to do with maneuverability, either due to the nature of work or the need to stay in a channel to prevent running aground. Of course you can’t expect a tanker to break/turn on a dime, but there’s nothing in the rules about tonnage.