Washington State Ferries cover a wide range of routes. Some of them are major commuter journeys and the boats are sized to handle lots of cars and passengers. Others are far more specialized runs that have lower frequencies and capacity. This can change throughout the year with the peak season demand being way higher than the off-season and larger boats get drafted in.
While I was on the shore at Coupeville, one of the ferries came in. We actually took this route on a vacation a couple of years ago and it was quite busy then. This time things were a lot quieter and the boat was clearly not one of the biggest that WSDOT has available.