you have no gain by doing so, since you do not have more information. Low level example: you have a bit so it can be one or zero: b = [0 1]. Now you store that bit in 2 bit field. It will still have the value 0 or 1, so the rest of the bitfield is simply unused since there is no information to fill it with. The same for your image example: you have 2^20 colors you want to sort them in 2^21 bins. this leaves some of the bins empty. no gain here.
you can use methodes to fill those level, like interpolation but this will not make additional information "appear".