This article provides information on the V-Ray 6 for Rhino textures compression changes.
In V-Ray 5 Windows GDI+ image compressor was used to convert .tx files to .jpg, which has a default quality level of 75 (meaning 25% compression). While in V-Ray 6 it was changed to V-Ray texture sampler using libjpeg image compressor, which has a default quality level of 96 (4% compression).
This ensures better texture quality in the Rhino viewport. As a result embedded texture files may increase the model file size compared to previous V-Ray versions.
With V-Ray 6.20.00, V-Ray procedural textures are displayed in the Rhino viewport by baking the texture into a temporary image file. By default, the baked viewport preview of bitmap texture matches the bitmap resolution. This resolution can be changed via the vrayTexturePreviewSettings command.
Note: the command does not update existing textures, changes will apply the next time a texture is changed. In order to update all textures in the scene, run the command, apply new values, and clear the texture cache folder which is located in: %temp%\McNeel\Rhino\V7\TextureCache. Once it is cleared, resave the file - this will decrease file size at the expense of the quality.