The internal format for the RGB texture, by default, is not actually RGB, but is instead DXT1 compression, with 64 bits per 4x4 block of pixel (or 4 bits per pixel.)
The internal format for the grayscale texture, by default, is a straight 8 bits per pixel grayscale format.
Hence, the RGB input ends up being smaller at runtime, with default settings. In fact, exactly half.
Note that there’s also overhead for mip maps, and for the general file header/asset information, although those are generally smaller than the main payload image.