If the firmware wanted to, it could actually make a lot of raw images even smaller than loss-y cRAW without any loss, but the highest compression levels take a significant amount of time to do, and that would slow the fps considerably. I remember once converting cr2 files to uncompressed DNG, where the raw data sits inside DNG as an uncompressed TIF, and used 7Zip to compress the images the best it could, and some were down to around 20% the size of the compressed cr2.
"Exposure to the left", if one is comfortable with the possible extra read noise, makes images easier to compress, since it leaves the most significant bits highly contiguous in the image, in the same color channel. When you "expose to the right" the least significant bits are all very busy with noise.