RAW imagery I have worked with is about 25-50% losslessly compressible on average. Most of your gains in image compression are from quantizing the gamut in clever, (usually) imperceptible ways.
Raw imagery contains a lot of entropy that usually doesn't affect the appearance perceptually, but still has sig figs that frustrate compression.
Raw imagery contains a lot of entropy that usually doesn't affect the appearance perceptually, but still has sig figs that frustrate compression.