No, they're not, if a figure obtains the same shape, and size doesn't mean they're similar. Even though they have two similarities, volume and mass come into play. Figures of all shapes and sizes, even those that are the same, can embody a different volume and mass.
Example: Toni has a sphere that's the same size of sphere B. Toni has to figure out why one sphere is different from the other. What are some reasons why the spheres are not similar?
Well, the objects can be made out of different materials. Sphere A could be made out of wood while Sphere B could be made out of copper. Same object, same size, different mass, and volumes. As we know, copper is typically heavier than wood, so, it really all comes down to what the object is created with.
I hope I helped :)