They are different in design, physical size, detail, and of course their electronic size
A model ( SANIX3D ) can vary between 20 mb, up to 2 – 3 gb.
The optimization operation is not at all simple … It is necessary to convert a 3gb model into a
100 – 200 mb.
The size of 100 mb is average for a STL file for 3D Printing, here it depends on the PC you have it, and the SLICER that you use it
So some models are split into a few parts during design, and then optimized.
In SANIX3D models, quality is not lost after optimization, so some projects, at the end of the design, do not have a solid version
In the images below, you see 2 pictures, where the difference is shown.

When designing a project with the purpose of high-detail and preserving it for printing, the print area of the files is taken into consideration.

If the details are designed for a model that needs to be printed at a minimum scale of 1:16 but SANIX3D recommends a scale of 1:10, in this case, it cannot be printed, for example, at a scale of 1:32 because it will lose approximately 50% of the model’s details. SANIX3D’s goal is not to provide projects where not all the details can be printed.

SANIX3D does not offer miniatures in its current projects (with certain exceptions in the projects already available on www.sanix3d.com  )

Many people who desire the solid version of the models are those who want to create their own renders of the model, but this is not allowed.

In points 1.7 of the license, this is indicated:

General Terms and Conditions