There are plenty of terms for describing the size of a ship. However, most of them are either measures of cargo capacity, or measures of volume. To make things more confusing, most of the volume measures use weight-sounding names such as "ton".
If I were to pick up a ship and place it on a scale, what is the technical term I would use to describe the number I got? Given that seafaring has its own specialized vocabulary, I'm pretty sure I'd get laughed at if I said "the Seawise Giant weighs x million pounds".