I have an EXR lightmap that weighs 9 MB, but in the texture inspector it shows 16 MB ( true color no compression), after disabling mipmaps it falls to 12 MB, Why the 3 MB difference? shouldn’t I see 9 MB just like its original size?
The inspector displays the texture size “in game” (in memory etc.), not the source file (which is only used in the editor) size.
E.g. if you have a .JPG texture on disk, it might be fairly small because JPG files are compressed in a special way. But JPG compression can not be used by the graphics card. So Unity converts all textures into an internal format, with optional GPU-friendly texture compression (DXT and so on).
In other words, size of the file you use in the editor has no related to size of texture as it will be in your game data file or in memory.