Using the Rescale World Units Utility is not the same as using the Scale transform tool, which scales the geometry by transforming it. For example, if you have three spheres, each with a radius of 25, and you scale the world units of the second sphere, and then use the Scale transform tool to double the size of the third sphere, the first sphere will look half the size of the other two spheres, and its Radius spinner will read 25. The second sphere will look twice as big as the first, and its Radius spinner will read 50 (because its world units have been doubled), and the third sphere will be the same size as the second, twice as big as the first, but its Radius spinner will read 25, because you’ve scaled the transformation of the geometry in the Stack after the creation parameters that establish the original radius of the sphere.
Another difference between scaling the world units of an object and performing a Scale transform is that a Scale transform affects only the geometry, while scaling the world units affects the scale of everything that has to do with the object, including the scale of assigned materials that might have 3D maps. For example, if you use Scale transform to double the size of a sphere assigned a Marble mapped material with a marble size of 20, the mapped effect on the rendered sphere appears twice as large, but its Size parameter remains 20. If you double the world units of the sphere, however, the mapped effect in the rendering also appears twice as large, but the Size parameter is also doubled to 40.
Caution: If you scale only a selected portion of the scene, and both the selected and unselected items contain instanced components (such as 3D maps), you can run into some discrepancies. As a specific example, create two spheres, each assigned the same material using a Noise map. Render the two spheres, and clone the Virtual Display Buffer. Now, select and rescale the world units of one of the spheres to a Scale Factor of 2. Render the result and compare the two renderings. The altered sphere is twice the size of the other and its noise pattern has been doubled, so it covers the same relative surface. The noise pattern on the first sphere, however is different from that in the first rendering because the Size setting of the 3D Noise map was doubled by the rescaling, and the map is instanced in the scene and dependent on world units rather than local object coordinates. In this case, you could avoid the problem by making sure that the 3D maps assigned to the rescaled objects are all unique.
There’s a new option in the General page of the File/Preferences dialog that also affects world unit scale.