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In answer to 1. Indexed formats are always more efficient that regular formats. This is why GIF is always indexed. With an index the color is only defined once and then referenced multiple times. I suspect there is a need for more than one byte to direct reference a pixel even though you only have 256 shades. However, I am not sure of the internal formats used in PNG. You might try reducing below 256 shades and see if the ratio changes.
Some sort of crunching would seem in order. PNG seems not to compress itself very well.
Dale
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