@mcc presumably yes, though if the compression is working well I expect it doesn't make much difference
As in, if every pixel is 373737 or ababab then that's exactly the kind of thing that zlib ought to be very good at identifying runs of and copying from history
We had a similar experience when adding PNG screenshot support to Chocolate Doom, where we found that scaling screenshots up from 320x200 to 1600x1200 (to get aspect ratio correct) had almost no effect on file size