New answers tagged compression
Although the question was asked about Adobe Photoshop, the behavior is due to the lossy JPEG format and would be similar with any image editor. Cropping a JPEG can make it less compressible, especially when the x and y offsets of the cropped area are odd numbers. This causes a re-subsampling of the color channels that can make the cropped image more complex ...
Blending modes do not translate to PNG images. I would guess that you've se the shadow to Multiply within Photoshop. Your shadows need to be set to Normal for the blending mode. This may mean you need to better mask the shadow layer or remove the shadow entirely and recreate it without the white areas.
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