Is it just the subsampling?
Naively i would assume that even if you have the unnessary the C_b C_r channels in a greyscale image, they are going to compress really well since they have very little information in a greyscale image.
Naively i would assume that even if you have the unnessary the C_b C_r channels in a greyscale image, they are going to compress really well since they have very little information in a greyscale image.