UK wave power far too costly, page-31

The magnitude of the buoyant force on an object is equal to the weight of the fluid it displaces.
As with other forces, the SI unit of buoyancy is the newton [N].
Buoyancy is caused by differences in pressure acting on opposite sides of an object immersed in a static fluid.

• The factors that affect buoyancy are…
  • the density of the fluid,
  • the volume of the fluid displaced, and
  • the local acceleration due to gravity.
• The buoyant force is not affected by…
  • the mass of the immersed object or
  • the density of the immersed object.

I am also no Engineer however would suggest that you do not necessarily need 4 times the displacement to achieve for times the force (CETO 5 to CETO 6).... therefore the CET6 is sized accordingly.
Good old Google Manuals