The confusion seems to arise from the way in which some software displays file sizes. Some actually refer to MB, when they actually mean MP.
Looking at a detailed Properties list for each file type, I see that images shot as RAW files have an actual file size of 27.66MB (29,000,049) and are 24MP resolution. Images shot as JPG's are 18.5MB (19,394,782) with the same resolution.
If I open an embedded JPG from within a RAW file, then save it directly, it has a file size of 5.9MB and a resolution of 24MP. Given that the camera uses its own software to convert the RAW file to an embedded JPG, it would seem that this does involve some significant compression.
Having got that far, I'm not quite so confused as I was previously and can see that the answer to my original question is that the embedded JPG is not a direct or adequate substitute for one shot directly.
Phew!
I don't see why it is not adequate as a substitute, other then the extra effort to acquire it. That is the idea of it, the same image makes both the embedded and the JPG file. The extracted embedded image does not have Exif data (if from ExifTool), but the Exif is still in the raw file.
But image size is apples and oranges. MB is NEVER a way to compare image size. Image size is dimensioned in pixels.
RGB images (JPG and TIF for example) are typically 24 bit color (JPG color always is 24 bits), which is
3 bytes RGB per pixel.
So RGB 24 mp is 24x3 = 72 million bytes, every time (if 24 bit RGB color).
However (the confusion), then file data compression reduces the storage size, tremendously in JPG, and (optionally) significantly in TIF or raw files. JPG file size might be 1/4 to probably 1/8 to maybe 1/12 of data size. It has the dickens squeezed out of it (with losses, JPG compression is Not lossless). An image of the SAME pixel dimensions, but in larger JPG file, because it uses less compression, is a better quality file than a smaller JPG file of same pixel dimensions. Then, when the JPG file is opened into computer memory, it is always 3 bytes per pixel again (assuming standard 24 bit color, which all JPG is).
Raw files are NOT RGB data, and instead are Bayer GRGB 12 bits or 14 bits per pixel, so data size is more like 1.5 bytes per pixel. Again, file data compression reduces this number for a smaller file, significantly, but not as dramatically as JPG. We cannot view raw images, so any raw image presented for us to view and see has in fact been converted to RGB, for our RGB LCD monitors to be able to use. This is WHY the embedded JPG, to provide a means to see it on the camera rear LCD monitor, and also it is the histogram that we view. However, the JPG we convert later in raw software may not exactly match the same options specified in the embedded JPG that the camera converted (the idea is that we can make it even better).
