In any case, the embedded JPG is a very variable thing in function of the brand and the model. With my first Nikon cameras, the embedded JPG was very small. With the latter models, it grew larger, up to full size.

It is also possible that more than one JPG file is embedded. For instance, looking at one of the latest .NEF I’ve shot (Nikon D7000) with exiftool, I see:

Jpg From Raw : (Binary data 1005709 bytes, use -b option to extract)
Preview Image : (Binary data 100897 bytes, use -b option to extract)

The JpgFromRaw is 4928×3264, while the RAW image size is 4992×3280 – this is the usual clipping of a few pixels at the borders, typically used for noise or black-level compensation. Thus we can consider a full-size JPG. The PreviewImage is clearly smaller.

For what concerns my Sony NEX-6:

Preview Image : (Binary data 869317 bytes, use -b option to extract)
Thumbnail Image : (Binary data 10417 bytes, use -b option to extract)

The PreviewImage here turns out to be 1616×1080 pixels, while the RAW is 4912×3264, so it looks like there’s no full size JPG (in fact exiftool doesn’t call it “JpgFromRaw”), in spite of the fact that the NEX-6 can be probably considered prosumer. The ThumbnailImage here is tiny.

There are 8 bits to a byte.