SaveSmall is a save option (and command) to save without render meshes.

Rhino is a NURBS modeller, though shaded and rendered views are based on so called 'render meshes'.

You can find the settings for these meshes under: file/properties/mesh

This is the reason that sometimes geometry is mathematically correct but doesn't show well in Rhino shaded viewport or rendered picture.

Render meshes can increase the file size heavily. Therefore it can be smart to SaveSmall when files are send via e-mail. When a file is saved small it takes a while to regenerate the render meshes, corresponding to the last saved mesh settings, after re-opening the file.