Why DNG files are bigger when converting from TIFF?

when shoot in raw, normal workflow convert images dng , start editing in lightroom. know, there photos need editing in pixel based software such photoshop.

 

when right-click dng in lightroom , chose "edit in... » edit in adobe photoshop", photo opened in photoshop 8-bit tiff file editing. demonstration of issue, let's assume don't changes , save file is. new tiff file created next source dng "-edit" in it's name.

 

back lightroom, both files same, 1 dng, other in 8-bit tiff. point (assuming did change in photoshop, otherwise point in opening there) should further editing in lightroom in tiff , not dng one. let's i'm done work , export final edited tiff file dng (i format , keep metadata changes can revert original). i've exported original dng file comparison.

 

and realize following:

dng dng » ~7mb (basically same original dng)

tiff dng » ~15mb

 

why such big difference?

 

i didn't editing either in original dng or photoshop converted tiff. techincal reason i'm not aware or doing wrong?

there 2 things can think of cause it. 1)tiff 8 bit , dng either 16 or 32 bit. 2) tiff file embedded inside dng file.

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